Multi-layer cover golf ball having a high acid casing layer

ABSTRACT

A golf ball includes a single core having an outer surface and a geometric center. The core is formed from a substantially homogenous rubber composition. An inner cover layer is disposed about the core, the inner cover including a high-acid ionomer and having a material hardness of about 66 to 75 Shore D. The high-acid ionomer has an acid content of about 16% or greater. An outer cover layer is disposed about the inner cover layer, the outer cover including a polyurethane and having a material hardness of about 38 Shore D to about 56 Shore D. The core surface hardness is from 0 Shore C to 15 Shore C lower than the geometric center hardness to define a hardness gradient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 14/144,449, filed Dec. 30, 2013, which is acontinuation-in-part of U.S. patent application Ser. No. 13/723,469,filed Dec. 21, 2012 and now U.S. Pat. No. 8,845,456, which is acontinuation of U.S. patent application Ser. No. 13/438,947, filed Apr.4, 2012 and now U.S. Pat. No. 8,337,330, which is a continuation of U.S.patent application Ser. No. 12/964,343, filed Dec. 9, 2010 and now U.S.Pat. No. 8,152,655, which is a continuation-in-part of U.S. patentapplication Ser. No. 12/647,584, filed Dec. 28, 2009 and now U.S. Pat.No. 8,500,575, which is a continuation-in-part of U.S. patentapplication Ser. No. 12/558,826, filed Sep. 14, 2009 and now U.S. Pat.No. 7,857,714, which is a continuation of U.S. patent application Ser.No. 12/186,877, filed Aug. 6, 2008 and now U.S. Pat. No. 7,803,069,which is a continuation of U.S. patent application Ser. No. 11/832,197,filed Aug. 1, 2007 and now U.S. Pat. No. 7,410,429, which is acontinuation-in-part of U.S. patent application Ser. No. 11/829,461,filed Jul. 27, 2007 and now U.S. Pat. No. 7,537,530, which is acontinuation-in-part of U.S. patent application Ser. No. 11/772,903,filed Jul. 3, 2007 and now U.S. Pat. No. 7,537,529, the disclosures ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to golf balls with cores having one ormore layers, any of the layers having a ‘negative’ or ‘positive’hardness gradient, trans gradient, or both. More particularly, the golfball has a core of one or more layers where at least one layer,preferably a single, solid core, has a low, “positive” hardness gradientand concurrently includes a trans-polybutadiene gradient.

BACKGROUND OF THE INVENTION

Solid golf balls are typically made with a solid core encased by acover, both of which can have multiple layers, such as a dual corehaving a solid center and an outer core layer, or a multi-layer coverhaving an inner. Generally, golf ball cores and/or centers areconstructed with a thermoset rubber, typically a polybutadiene-basedcomposition. The cores are usually heated and crosslinked to createcertain characteristics, such as higher or lower compression, which canimpact the spin rate of the ball and/or provide better “feel.” These andother characteristics can be tailored to the needs of golfers ofdifferent abilities. From the perspective of a golf ball manufacturer,it is desirable to have cores exhibiting a wide range of properties,such as resilience, durability, spin, and “feel,” because this enablesthe manufacturer to make and sell many different types of golf ballssuited to differing levels of ability.

Heretofore, most single core golf ball cores have had a conventionalhard-to-soft hardness gradient from the surface of the core to thecenter of the core, otherwise known as a “positive hardness gradient.”These gradients, however, are typically quite large, upwards of 15, 20,even 25 or more Shore C hardness points. The patent literature containsa number of references, additionally, that discuss ahard-surface-to-soft-center hardness gradient across a golf ball core.

U.S. Pat. No. 4,650,193 to Molitor et al. generally discloses a hardnessgradient in the surface layers of a core by surface treating a slug ofcurable elastomer with a cure-altering agent and subsequently moldingthe slug into a core. This treatment allegedly creates a core with twozones of different compositions, the first part being the hard,resilient, central portion of the core, which was left untreated, andthe second being the soft, deformable, outer layer of the core, whichwas treated by the cure-altering agent. The two “layers” or regions ofthe core are integral with one another and, as a result, achieve theeffect of a gradient of soft surface to hard center.

U.S. Pat. No. 3,784,209 to Berman, et al. generally discloses asoft-to-hard hardness gradient. The '209 patent discloses anon-homogenous, molded golf ball with a core of “mixed” elastomers. Acenter sphere of uncured elastomeric material is surrounded by acompatible but different uncured elastomer. When both layers ofelastomer are concurrently exposed to a curing agent, they becomeintegral with one another, thereby forming a mixed core. The center ofthis core, having a higher concentration of the first elastomericmaterial, is harder than the outer layer. One drawback to this method ofmanufacture is the time-consuming process of creating first elastomerand then a second elastomer and then molding the two together.

Other patents discuss cores that receive a surface treatment to providea soft ‘skin’. However, since the interior portions of these cores areuntreated, they have the similar hard surface to soft center gradient asconventional cores. For example, U.S. Pat. No. 6,113,831 to Nesbitt etal. generally discloses a conventional core and a separate soft skinwrapped around the core. This soft skin is created by exposing thepreform slug to steam during the molding process so that a maximum moldtemperature exceeds a steam set point, and by controlling exothermicmolding temperatures during molding. The skin comprises theradially-outermost 1/32 inch to ¼ inch of the spherical core. U.S. Pat.Nos. 5,976,443 and 5,733,206, both to Nesbitt et al., disclose theaddition of water mist to the outside surface of the slug before moldingin order to create a soft skin. The water allegedly softens thecompression of the core by retarding crosslinking on the core surface,thereby creating an even softer soft skin around the hard centralportion.

Additionally, a number of patents disclose multi-layer golf ball cores,where each core layer has a different hardness thereby creating ahardness gradient from core layer to core layer.

There remains a need, however, for a multi-layer golf ball having singlelayer core that has a shallow hard-to-soft (“positive”) hardnessgradient, from the surface to the center, and to achieve a method ofproducing such a core that is inexpensive and efficient.

SUMMARY OF THE INVENTION

The present invention is directed to a golf ball including a single coreand a double cover. The single core has an outer surface and a geometriccenter, and is formed from a substantially homogenous rubbercomposition. The double cover includes an inner cover layer and an outercover layer. The inner cover layer, which is disposed about the core, isformed from an ionomeric material. The inner cover layer has a materialhardness of about 60 Shore D or greater. The outer cover layer isdisposed about the inner cover layer and is typically formed from acastable polyurea or a castable polyurethane. This layer has a materialhardness of about 60 Shore D or less. In one preferred embodiment, theouter surface of the core has a trans content of about 12% or less and ahardness of about 71 to 88 Shore C, the geometric center has a transcontent of about 10% or less and a hardness of about 70 to 80 Shore C,and the core surface hardness is greater than the geometric centerhardness by about 1 to 10 Shore C to define a shallow positive hardnessgradient.

In one embodiment, the positive hardness gradient is about 2 to 8 ShoreC, more preferably about 2.5 to 7 Shore C. The core preferably has anouter diameter of about 1.45 to about 1.62 inches, more preferably about1.50 to about 1.58 inches. The hardness of the geometric center of thecore is generally about 72 to 78 Shore C and the hardness of the coresurface is generally about 73 to 80 Shore C, more preferably about 74 to78 Shore C. In another preferred embodiment, the outer surface of thecore has a trans content of about 10% or less and the geometric centerof the core has a trans content of about 8% or less.

The present invention is also directed to a golf ball formed from asingle core having an outer diameter of about 1.5 to 1.55 inches. Thecore is typically formed from a substantially homogenous rubbercomposition. An inner cover layer is disposed about the core and istypically formed from a thermoplastic ionomeric material having amaterial hardness of about 66 to 80 Shore D. An outer cover layer isdisposed about the inner cover layer and has a thickness of about 0.04inches. In this embodiment, the outer cover layer is formed from anionomer having a material hardness of about 64 to 68 Shore D, or athermosetting castable polyurethane having a hardness of about 51 to 55Shore D. Preferably, the outer surface of the core has a trans contentof about 12% or less and a hardness of about 74 to 78 Shore C, thegeometric center has a trans content of about 10% or less and a hardnessof about 72 to 78 Shore C, and the core surface hardness is greater thanthe geometric center hardness by about 1 to 5 Shore C to define ashallow positive hardness gradient, the core having an Atti compressionof about 50 to 70.

In one embodiment, the golf ball further includes an intermediate layerdisposed between the inner cover layer and the core, the intermediatelayer having a thickness of about 0.06 inches and a hardness of about 43to 47 Shore D. Generally, the inner cover hardness is about 66 Shore D,the intermediate layer hardness is about 45 Shore D, and the outer coverlayer hardness is about 53 Shore D.

The present invention is further directed to a golf ball including asingle core having an outer diameter of about 1.45 inches, an outersurface, and a geometric center, and being formed from a substantiallyhomogenous rubber composition. An inner cover layer is disposed aboutthe core. The inner cover has an outer diameter of about 1.58 inches andis formed from a thermoplastic non-ionomeric material having a materialhardness of about 38 to 42 Shore D. An outer cover layer is disposedabout the inner cover layer. The outer cover has a thickness of about0.05 inches and is formed from an ionomer having a material hardness ofabout 66 to 70 Shore D. Preferably, the outer surface of the core has atrans content of about 12% or less and a hardness of about 74 to 78Shore C, the geometric center has a trans content of about 10% or lessand a hardness of about 72 to 78 Shore C, and the core surface hardnessis greater than the geometric center hardness by about 1 to 5 Shore C todefine a shallow positive hardness gradient.

The present invention is directed to a golf ball having a single core.The core has an outer surface and a geometric center and is formed froma substantially homogenous rubber composition. An inner cover layersurrounds the core, the inner cover including a high-acid ionomer andhaving a material hardness of about 66 to 75 Shore D. The high-acidionomer typically has an acid content of about 16 wt. % or greater. Anouter cover layer surrounds the inner cover layer. Preferably, the outercover is formed from a polyurethane material and has a material hardnessof about 38 Shore D to about 56 Shore D. The core surface hardness isgenerally from about 0 Shore C to 15 Shore C lower than the geometriccenter hardness to define a hardness gradient.

In one embodiment, the ionomer has an acid content of about 18.5% toabout 21.5%. The core has an outer diameter of about 1.45 to about 1.62inches, more preferably about 1.50 to about 1.58 inches. The hardness ofthe geometric center of the core is about 60 to 80 Shore C, morepreferably about 72 to 78 Shore C, and the hardness of the surface ofthe core is about 73 to 80 Shore C, more preferably about 74 to 78 ShoreC. In a preferred embodiment, the hardness of the outer surface of thecore is about 1 Shore C to 10 Shore C lower than the hardness at thegeometric center. In an alternative embodiment, the hardness of theouter surface of the core is 1 Shore C to 5 Shore C lower than thehardness at the geometric center.

The present invention is directed to a golf ball having a single core.The core has an outer surface and a geometric center and is formed froma substantially homogenous rubber composition. An inner cover layersurrounds the core, the inner cover including a high-acid ionomer andhaving a material hardness of about 66 to 75 Shore D. The high-acidionomer typically has an acid content of about 16 wt. % or greater. Anouter cover layer surrounds the inner cover layer. Preferably, the outercover is formed from a polyurethane material and has a material hardnessof about 38 Shore D to about 56 Shore D. The core surface hardness isgreater than the geometric center hardness by about 1 to 10 Shore C todefine a shallow positive hardness gradient. In a preferred embodiment,the positive hardness gradient is about 1 to 5 Shore C. In analternative embodiment, the positive hardness gradient is about 2 to 8Shore C, more preferably about 2.5 to 7 Shore C.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention may be more fullyunderstood with reference to, but not limited by, the followingdrawings.

FIG. 1 is a representative cross section of a golf ball of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The golf balls of the present invention may include a single-layer(one-piece) golf ball, and multi-layer golf balls, such as one having acore and a cover surrounding the core, but are preferably formed from acore comprised of a solid center (otherwise known as an inner core) andan outer core layer, an inner cover layer and an outer cover layer. Ofcourse, any of the core and/or the cover layers may include more thanone layer. In a preferred embodiment, the core is formed of an innercore and an outer core layer where both the inner core and the outercore layer have a “soft-to-hard” hardness gradient (a “negative”hardness gradient) radially inward from each component's outer surfacetowards its innermost portion (i.e., the center of the inner core or theinner surface of the outer core layer), although alternative embodimentsinvolving varying direction and combination of hardness gradient amongstcore components are also envisioned (e.g., a “negative” gradient in thecenter coupled with a “positive” gradient in the outer core layer, orvice versa).

The center of the core may also be a liquid-filled or hollow spheresurrounded by one or more intermediate and/or cover layers, or it mayinclude a solid or liquid center around which tensioned elastomericmaterial is wound. Any layers disposed around these alternative centersmay exhibit the inventive core hardness gradient (i.e., “negative”). Thecover layer may be a single layer or, for example, formed of a pluralityof layers, such as an inner cover layer and an outer cover layer.

As briefly discussed above, the inventive cores may have a hardnessgradient defined by hardness measurements made at the surface of theinner core (or outer core layer) and radially inward towards the centerof the inner core, typically at 2-mm increments. As used herein, theterms “negative” and “positive” refer to the result of subtracting thehardness value at the innermost portion of the component being measured(e.g., the center of a solid core or an inner core in a dual coreconstruction; the inner surface of a core layer; etc.) from the hardnessvalue at the outer surface of the component being measured (e.g., theouter surface of a solid core; the outer surface of an inner core in adual core; the outer surface of an outer core layer in a dual core,etc.). For example, if the outer surface of a solid core has a lowerhardness value than the center (i.e., the surface is softer than thecenter), the hardness gradient will be deemed a “negative” gradient (asmaller number−a larger number=a negative number). It is preferred thatthe inventive cores have a zero or a negative hardness gradient, morepreferably between zero (0) and −10, most preferably between 0 and −5.

Preferably, the core layers (inner core or outer core layer) is madefrom a composition including at least one thermoset base rubber, such asa polybutadiene rubber, cured with at least one peroxide and at leastone reactive co-agent, which can be a metal salt of an unsaturatedcarboxylic acid, such as acrylic acid or methacrylic acid, anon-metallic coagent, or mixtures thereof. Preferably, a suitableantioxidant is included in the composition. An optional soft and fastagent (and sometimes a cis-to-trans catalyst), such as an organosulfuror metal-containing organosulfur compound, can also be included in thecore formulation

Other ingredients that are known to those skilled in the art may beused, and are understood to include, but not be limited to,density-adjusting fillers, process aides, plasticizers, blowing orfoaming agents, sulfur accelerators, and/or non-peroxide radicalsources.

The base thermoset rubber, 130 which can be blended with other rubbersand polymers, typically includes a natural or synthetic rubber. Apreferred base rubber is 1,4-polybutadiene having a cis structure of atleast 40%, preferably greater than 80%, and more preferably greater than90%. Examples of desirable polybutadiene rubbers include BUNA® CB22 andBUNA® CB23, CB1221, CB1220, CB24, and CB21, commercially-available fromLANXESS Corporation; UBEPOL® 360L and UBEPOL® 150L and UBEPOL-BRrubbers, commercially available from UBE Industries, Ltd. of Tokyo,Japan; KINEX® 7245, KINEX® 7265, and BUDENE 1207 and 1208, commerciallyavailable from Goodyear of Akron, Ohio; SE BR-1220; Europrene® NEOCIS®BR 40 and BR 60, commercially available from Polimeri Europa; and BR 01,BR 730, BR 735, BR 11, and BR 51, commercially available from JapanSynthetic Rubber Co., Ltd; PETROFLEX® BRNd-40; and KARBOCHEM® ND40,ND45, and ND60, commercially available from Karbochem.

From the Lanxess Corporation, most preferred are the neodymium andcobalt catalyzed grades, but all of the following may be used: Buna CB21; Buna CB 22; Buna CB 23; Buna CB 24; Buna CB 25; Buna CB 29 MES; BunaCB Nd 40; Buna CB Nd 40 H; Buna CB Nd 60; Buna CB 55 NF; Buna CB 60;Buna CB 45 B; Buna CB 55 B; Buna CB 55 H; Buna CB 55 L; Buna CB 70 B;Buna CB 1220; Buna CB 1221; Buna CB 1203; Buna CB 45. Additionally,numerous suitable rubbers are available from JSR (Japan SyntheticRubber), Ubepol sold by Ube Industries Inc, Japan, BST sold by BSTElastomers, Thailand; IPCL sold by Indian Petrochemicals Ltd, India;Nitsu sold by Karbochem or Karbochem Ltd of South Africa; Petroflex ofBrazil; LG of Korea; and Kuhmo Petrochemical of Korea.

The base rubber may also comprise high or medium Mooney viscosityrubber, or blends thereof. A “Mooney” unit is a unit used to measure theplasticity of raw or unvulcanized rubber. The plasticity in a “Mooney”unit is equal to the torque, measured on an arbitrary scale, on a diskin a vessel that contains rubber at a temperature of 100° C. and rotatesat two revolutions per minute. The measurement of Mooney viscosity isdefined according to ASTM D-1646.

The Mooney viscosity range is preferably greater than about 40, morepreferably in the range from about 40 to about 80 and more preferably inthe range from about 40 to about 60. Polybutadiene rubber with higherMooney viscosity may also be used, so long as the viscosity of thepolybutadiene does not reach a level where the high viscositypolybutadiene clogs or otherwise adversely interferes with themanufacturing machinery. It is contemplated that polybutadiene withviscosity less than 65 Mooney can be used with the present invention.Gin one embodiment of the present invention, glf ball cores made withmid- to high-Mooney viscosity polybutadiene material exhibit increasedresiliency (and, therefore, distance) without increasing the hardness ofthe ball. Such cores are soft, i.e., compression less than about 60 andmore specifically in the range of about 50-55. Cores with compression inthe range of from about 30 about 50 are also within the range of thispreferred embodiment.

Commercial sources of suitable mid- to high-Mooney viscositypolybutadiene include Bayer AG CB23 (Nd-catalyzed), which has a Mooneyviscosity of around 50 and is a highly linear polybutadiene, and CB1221(Co-catalyzed). If desired, the polybutadiene can also be mixed withother elastomers known in the art, such as other polybutadiene rubbers,natural rubber, styrene butadiene rubber, and/or isoprene rubber inorder to further modify the properties of the core. When a mixture ofelastomers is used, the amounts of other constituents in the corecomposition are typically based on 100 parts by weight of the totalelastomer mixture. In one preferred embodiment, the base rubbercomprises a Nd-catalyzed polybutadiene, a rare earth-catalyzedpolybutadiene rubber, or blends thereof. If desired, the polybutadienecan also be mixed with other elastomers known in the art such as naturalrubber, polyisoprene rubber and/or styrene-butadiene rubber in order tomodify the properties of the core. Other suitable base rubbers includethermosetting materials such as, ethylene propylene diene monomerrubber, ethylene propylene rubber, butyl rubber, halobutyl rubber,hydrogenated nitrile butadiene rubber, nitrile rubber, and siliconerubber.

Thermoplastic elastomers (TPE) many also be used to modify theproperties of the core layers, or the uncured core layer stock byblending with the base thermoset rubber. These TPEs include natural orsynthetic balata, or high trans-polyisoprene, high trans-polybutadiene,or any styrenic block copolymer, such as styrene ethylene butadienestyrene, styrene-isoprene-styrene, etc., a metallocene or othersingle-site catalyzed polyolefin such as ethylene-octene, orethylene-butene, or thermoplastic polyurethanes (TPU), includingcopolymers, e.g. with silicone. Other suitable TPEs for blending withthe thermoset rubbers of the present invention include PEBAX®, which isbelieved to comprise polyether amide copolymers, HYTREL®, which isbelieved to comprise polyether ester copolymers, thermoplastic urethane,and KRATON®, which is believed to comprise styrenic block copolymerselastomers. Any of the TPEs or TPUs above may also contain functionalitysuitable for grafting, including maleic acid or maleic anhydride.

Additional polymers may also optionally be incorporated into the baserubber. Examples include, but are not limited to, thermoset elastomerssuch as core regrind, thermoplastic vulcanizate, copolymeric ionomer,terpolymeric ionomer, polycarbonate, polyamide, copolymeric polyamide,polyesters, polyvinyl alcohols, acrylonitrile-butadiene-styrenecopolymers, polyarylate, polyacrylate, polyphenylene ether,impact-modified polyphenylene ether, high impact polystyrene, diallylphthalate polymer, styrene-acrylonitrile polymer (SAN) (includingolefin-modified SAN and acrylonitrile-styrene-acrylonitrile polymer),styrene-maleic anhydride copolymer, styrenic copolymer, functionalizedstyrenic copolymer, functionalized styrenic terpolymer, styrenicterpolymer, cellulose polymer, liquid crystal polymer, ethylene-vinylacetate copolymers, polyurea, and polysiloxane or anymetallocene-catalyzed polymers of these species.

Suitable polyamides for use as an additional polymeric material incompositions within the scope of the present invention also includeresins obtained by: (1) polycondensation of (a) a dicarboxylic acid,such as oxalic acid, adipic acid, sebacic acid, terephthalic acid,isophthalic acid, or 1,4-cyclohexanedicarboxylic acid, with (b) adiamine, such as ethylenediamine, tetramethylenediamine,pentamethylenediamine, hexamethylenediamine, or decamethylenediamine,1,4-cyclohexanediamine, or m-xylylenediamine; (2) a ring-openingpolymerization of cyclic lactam, such as

-caprolactam or Ω-laurolactam; (3) polycondensation of anaminocarboxylic acid, such as 6-aminocaproic acid, 9-aminononanoic acid,11-aminoundecanoic acid, or 12-aminododecanoic acid; or (4)copolymerization of a cyclic lactam with a dicarboxylic acid and adiamine. Specific examples of suitable polyamides include NYLON 6, NYLON66, NYLON 610, NYLON 11, NYLON 12, copolymerized NYLON, NYLON MXD6, andNYLON 46.

Suitable peroxide initiating agents include dicumyl peroxide;2,5-dimethyl-2,5-di(t-butylperoxy) hexane;2,5-dimethyl-2,5-di(t-butylperoxy)hexyne;2,5-dimethyl-2,5-di(benzoylperoxy)hexane;2,2′-bis(t-butylperoxy)-di-iso-propylbenzene;1,1-bis(t-butylperoxy)-3,3,5-trimethyl cyclohexane; n-butyl4,4-bis(t-butyl-peroxy)valerate; t-butyl perbenzoate; benzoyl peroxide;n-butyl 4,4′-bis(butylperoxy) valerate; di-t-butyl peroxide; or2,5-di-(t-butylperoxy)-2,5-dimethyl hexane, lauryl peroxide, t-butylhydroperoxide, α-α bis(t-butylperoxy) diisopropylbenzene,di(2-t-butyl-peroxyisopropyl)benzene, di-t-amyl peroxide, di-t-butylperoxide. Preferably, the rubber composition includes from about 0.25 toabout 5.0 parts by weight peroxide per 100 parts by weight rubber (phr),more preferably 0.5 phr to 3 phr, most preferably 0.5 phr to 1.5 phr. Ina most preferred embodiment, the peroxide is present in an amount ofabout 0.8 phr. These ranges of peroxide are given assuming the peroxideis 100% active, without accounting for any carrier that might bepresent. Because many commercially available peroxides are sold alongwith a carrier compound, the actual amount of active peroxide presentmust be calculated. Commercially-available peroxide initiating agentsinclude DICUP™ family of dicumyl peroxides (including DICUP™ R, DICUP™40C and DICUP™ 40KE) available from Crompton (Geo Specialty Chemicals).Similar initiating agents are available from AkroChem, Lanxess,Flexsys/Harwick and R.T. Vanderbilt. Another commercially-available andpreferred initiating agent is TRIGONOX™ 265-50B from Akzo Nobel, whichis a mixture of 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane anddi(2-t-butylperoxyisopropyl) benzene. TRIGONOX™ peroxides are generallysold on a carrier compound.

Suitable reactive co-agents include, but are not limited to, metal saltsof diacrylates, dimethacrylates, and monomethacrylates suitable for usein this invention include those wherein the metal is zinc, magnesium,calcium, barium, tin, aluminum, lithium, sodium, potassium, iron,zirconium, and bismuth. Zinc diacrylate (ZDA) is preferred, but thepresent invention is not limited thereto. ZDA provides golf balls with ahigh initial velocity. The ZDA can be of various grades of purity. Forthe purposes of this invention, the lower the quantity of zinc stearatepresent in the ZDA the higher the ZDA purity. ZDA containing less thanabout 10% zinc stearate is preferable. More preferable is ZDA containingabout 4-8% zinc stearate. Suitable, commercially available zincdiacrylates include those from Sartomer Co. The preferred concentrationsof ZDA that can be used are about 10 phr to about 40 phr, morepreferably 20 phr to about 35 phr, most preferably 25 phr to about 35phr. In a particularly preferred embodiment, the reactive co-agent ispresent in an amount of about 29 phr to about 31 phr.

Additional preferred co-agents that may be used alone or in combinationwith those mentioned above include, but are not limited to,trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, andthe like. It is understood by those skilled in the art, that in the casewhere these co-agents may be liquids at room temperature, it may beadvantageous to disperse these compounds on a suitable carrier topromote ease of incorporation in the rubber mixture.

Antioxidants are compounds that inhibit or prevent the oxidativebreakdown of elastomers, and/or inhibit or prevent reactions that arepromoted by oxygen radicals. Some exemplary antioxidants that may beused in the present invention include, but are not limited to, quinolinetype antioxidants, amine type antioxidants, and phenolic typeantioxidants. A preferred antioxidant is2,2′-methylene-bis-(4-methyl-6-t-butylphenol) available as VANOX® MBPCfrom R.T. Vanderbilt. Other polyphenolic antioxidants include VANOX® T,VANOX® L, VANOX® SKT, VANOX® SWP, VANOX® 13 and VANOX® 1290.

Suitable antioxidants include, but are not limited to,alkylene-bis-alkyl substituted cresols, such as4,4′-methylene-bis(2,5-xylenol); 4,4′-ethylidene-bis-(6-ethyl-m-cresol);4,4′-butylidene-bis-(6-t-butyl-m-cresol);4,4′-decylidene-bis-(6-methyl-m-cresol);4,4′-methylene-bis-(2-amyl-m-cresol);4,4′-propylidene-bis-(5-hexyl-m-cresol);3,3′-decylidene-bis-(5-ethyl-p-cresol);2,2′-butylidene-bis-(3-n-hexyl-p-cresol);4,4′-(2-butylidene)-bis-(6-t-butyl-m-cresol);3,3′-4(decylidene)-bis-(5-ethyl-p-cresol);(2,5-dimethyl-4-hydroxyphenyl) (2-hydroxy-3,5-dimethylphenyl) methane;(2-methyl-4-hydroxy-5-ethylphenyl) (2-ethyl-3-hydroxy-5-methylphenyl)methane; (3-methyl-5-hydroxy-6-t-butylphenyl)(2-hydroxy-4-methyl-5-decylphenyl)-n-butyl methane;(2-hydroxy-4-ethyl-5-methylphenyl)(2-decyl-3-hydroxy-4-methylphenyl)butylamylmethane;(3-ethyl-4-methyl-5-hydroxyphenyl)-(2,3-dimethyl-3-hydroxy-phenyl)nonylmethane;(3-methyl-2-hydroxy-6-ethylphenyl)-(2-isopropyl-3-hydroxy-5-methyl-phenyl)cyclohexylmethane;(2-methyl-4-hydroxy-5-methylphenyl)(2-hydroxy-3-methyl-5-ethylphenyl)dicyclohexyl methane; and the like.

Other suitable antioxidants include, but are not limited to, substitutedphenols, such as 2-tert-butyl-4-methoxyphenol;3-tert-butyl-4-methoxyphenol; 3-tert-octyl-4-methoxyphenol;2-methyl-4-methoxyphenol; 2-stearyl-4-n-butoxyphenol;3-t-butyl-4-stearyloxyphenol; 3-lauryl-4-ethoxyphenol;2,5-di-t-butyl-4-methoxyphenol; 2-methyl-4-methoxyphenol;2-(1-methycyclohexyl)-4-methoxyphenol; 2-t-butyl-4-dodecyloxyphenol;2-(1-methylbenzyl)-4-methoxyphenol; 2-t-octyl-4-methoxyphenol; methylgallate; n-propyl gallate; n-butyl gallate; lauryl gallate; myristylgallate; stearyl gallate; 2,4,5-trihydroxyacetophenone;2,4,5-trihydroxy-n-butyrophenone; 2,4,5-trihydroxystearophenone;2,6-ditert-butyl-4-methylphenol; 2,6-ditert-octyl-4-methylphenol;2,6-ditert-butyl-4-stearylphenol; 2-methyl-4-methyl-6-tert-butylphenol;2,6-distearyl-4-methylphenol; 2,6-dilauryl-4-methylphenol;2,6-di(n-octyl)-4-methylphenol; 2,6-di(n-hexadecyl)-4-methylphenol;2,6-di(1-methylundecyl)-4-methylphenol;2,6-di(1-methylheptadecyl)-4-methylphenol;2,6-di(trimethylhexyl)-4-methylphenol;2,6-di(1,1,3,3-tetramethyloctyl)-4-methylphenol; 2-n-dodecyl-6-tertbutyl-4-methylphenol; 2-n-dodecyl-6-(1-methylundecyl)-4-methylphenol;2-n-dodecyl-6-(1,1,3,3-tetramethyloctyl)-4-methylphenol;2-n-dodecyl-6-n-octadecyl-4-methylphenol;2-n-dodecyl-6-n-octyl-4-methylphenol;2-methyl-6-n-octadecyl-4-methylphenol;2-n-dodecyl-6-(1-methylheptadecyl)-4-methylphenol;2,6-di(1-methylbenzyl)-4-methylphenol;2,6-di(1-methylcyclohexyl)-4-methylphenol;2,6-(1-methylcyclohexyl)-4-methylphenol;2-(1-methylbenzyl)-4-methylphenol; and related substituted phenols.

More suitable antioxidants include, but are not limited to, alkylenebisphenols, such as 4,4′-butylidene bis(3-methyl-6-t-butyl phenol);2,2-butylidene bis (4,6-dimethyl phenol); 2,2′-butylidenebis(4-methyl-6-t-butyl phenol); 2,2′-butylidene bis(4-t-butyl-6-methylphenol); 2,2′-ethylidene bis(4-methyl-6-t-butylphenol); 2,2′-methylenebis(4,6-dimethyl phenol); 2,2′-methylene bis(4-methyl-6-t-butyl phenol);2,2′-methylene bis(4-ethyl-6-t-butyl phenol); 4,4′-methylenebis(2,6-di-t-butyl phenol); 4,4′-methylene bis(2-methyl-6-t-butylphenol); 4,4′-methylene bis(2,6-dimethyl phenol); 2,2′-methylenebis(4-t-butyl-6-phenyl phenol);2,2′-dihydroxy-3,3′,5,5′-tetramethylstilbene; 2,2′-isopropylidenebis(4-methyl-6-t-butyl phenol); ethylene bis (beta-naphthol);1,5-dihydroxy naphthalene; 2,2′-ethylene bis (4-methyl-6-propyl phenol);4,4′-methylene bis(2-propyl-6-t-butyl phenol); 4,4′-ethylene bis(2-methyl-6-propyl phenol); 2,2′-methylene bis(5-methyl-6-t-butylphenol); and 4,4′-butylidene bis(6-t-butyl-3-methyl phenol);

Suitable antioxidants further include, but are not limited to, alkylenetrisphenols, such as 2,6-bis (2′-hydroxy-3′-t-butyl-5′-methylbenzyl)-4-methyl phenol; 2,6-bis (2′-hydroxy-3′-t-ethyl-5′-butylbenzyl)-4-methyl phenol; and 2,6-bis(2′-hydroxy-3′-t-butyl-5′-propylbenzyl)-4-methyl phenol.

The antioxidant is typically present in an amount of about 0.1 phr toabout 5 phr, preferably from about 0.1 phr to about 2 phr, morepreferably about 0.1 phr to about 1 phr. In a particularly preferredembodiment, the antioxidant is present in an amount of about 0.4 phr. Inan alternative embodiment, the antioxidant should be present in anamount to ensure that the hardness gradient of the inventive cores isnegative. Preferably, about 0.2 phr to about 1 phr antioxidant is addedto the core layer (inner core or outer core layer) formulation, morepreferably, about 0.3 to about 0.8 phr, and most preferably 0.4 to about0.7 phr. Preferably, about 0.25 phr to about 1.5 phr of peroxide ascalculated at 100% active can be added to the core formulation, morepreferably about 0.5 phr to about 1.2 phr, and most preferably about 0.7phr to about 1.0 phr. The ZDA amount can be varied to suit the desiredcompression, spin and feel of the resulting golf ball. The cure regimecan have a temperature range between from about 290° F. to about 335°F., more preferably about 300° F. to about 325° F., and the stock isheld at that temperature for at least about 10 minutes to about 30minutes.

The thermoset rubber composition of the present invention may alsoinclude an optional soft and fast agent. As used herein, “soft and fastagent” means any compound or a blend thereof that that is capable ofmaking a core 1) be softer (lower compression) at constant COR or 2)have a higher COR at equal compression, or any combination thereof, whencompared to a core equivalently prepared without a soft and fast agent.Preferably, the composition of the present invention contains from about0.05 phr to about 10.0 phr soft and fast agent. In one embodiment, thesoft and fast agent is present in an amount of about 0.05 phr to about3.0 phr, preferably about 0.05 phr to about 2.0 phr, more preferablyabout 0.05 phr to about 1.0 phr. In another embodiment, the soft andfast agent is present in an amount of about 2.0 phr to about 5.0 phr,preferably about 2.35 phr to about 4.0 phr, and more preferably about2.35 phr to about 3.0 phr. In an alternative high concentrationembodiment, the soft and fast agent is present in an amount of about 5.0phr to about 10.0 phr, more preferably about 6.0 phr to about 9.0 phr,most preferably about 7.0 phr to about 8.0 phr. In a most preferredembodiment, the soft and fast agent is present in an amount of about 2.6phr.

Suitable soft and fast agents include, but are not limited to,organosulfur or metal-containing organosulfur compounds, an organicsulfur compound, including mono, di, and polysulfides, a thiol, ormercapto compound, an inorganic sulfide compound, a Group VIA compound,or mixtures thereof. The soft and fast agent component may also be ablend of an organosulfur compound and an inorganic sulfide compound.

Suitable soft and fast agents of the present invention include, but arenot limited to those having the following general formula:

where R₁-R₅ can be C₁-C₈ alkyl groups; halogen groups; thiol groups(—SH), carboxylated groups; sulfonated groups; and hydrogen; in anyorder; and also pentafluorothiophenol; 2-fluorothiophenol;3-fluorothiophenol; 4-fluorothiophenol; 2,3-fluorothiophenol;2,4-fluorothiophenol; 3,4-fluorothiophenol; 3,5-fluorothiophenol2,3,4-fluorothiophenol; 3,4,5-fluorothiophenol;2,3,4,5-tetrafluorothiophenol; 2,3,5,6-tetrafluorothiophenol;4-chlorotetrafluorothiophenol; pentachlorothiophenol;2-chlorothiophenol; 3-chlorothiophenol; 4-chlorothiophenol;2,3-chlorothiophenol; 2,4-chlorothiophenol; 3,4-chlorothiophenol;3,5-chlorothiophenol; 2,3,4-chlorothiophenol; 3,4,5-chlorothiophenol;2,3,4,5-tetrachlorothiophenol; 2,3,5,6-tetrachlorothiophenol;pentabromothiophenol; 2-bromothiophenol; 3-bromothiophenol;4-bromothiophenol; 2,3-bromothiophenol; 2,4-bromothiophenol;3,4-bromothiophenol; 3,5-bromothiophenol; 2,3,4-bromothiophenol;3,4,5-bromothiophenol; 2,3,4,5-tetrabromothiophenol;2,3,5,6-tetrabromothiophenol; pentaiodothiophenol; 2-iodothiophenol;3-iodothiophenol; 4-iodothiophenol; 2,3-iodothiophenol;2,4-iodothiophenol; 3,4-iodothiophenol; 3,5-iodothiophenol;2,3,4-iodothiophenol; 3,4,5-iodothiophenol; 2,3,4,5-tetraiodothiophenol;2,3,5,6-tetraiodothiophenoland; and their zinc salts. Preferably, thehalogenated thiophenol compound is pentachlorothiophenol, which iscommercially available in neat form or under the tradename STRUKTOL®, aclay-based carrier containing the sulfur compound pentachlorothiophenolloaded at 45 percent (correlating to 2.4 parts PCTP). STRUKTOL® iscommercially available from Struktol Company of America of Stow, Ohio.PCTP is commercially available in neat form from eChinachem of SanFrancisco, Calif. and in the salt form from eChinachem of San Francisco,Calif. Most preferably, the halogenated thiophenol compound is the zincsalt of pentachlorothiophenol, which is commercially available fromeChinachem of San Francisco, Calif.

As used herein when referring to the invention, the term “organosulfurcompound(s)” refers to any compound containing carbon, hydrogen, andsulfur, where the sulfur is directly bonded to at least 1 carbon. Asused herein, the term “sulfur compound” means a compound that iselemental sulfur, polymeric sulfur, or a combination thereof. It shouldbe further understood that the term “elemental sulfur” refers to thering structure of S₈ and that “polymeric sulfur” is a structureincluding at least one additional sulfur relative to elemental sulfur.

Additional suitable examples of soft and fast agents (that are alsobelieved to be cis-to-trans catalysts) include, but are not limited to,4,4′-diphenyl disulfide; 4,4′-ditolyl disulfide; 2,2′-benzamido diphenyldisulfide; bis(2-aminophenyl) disulfide; bis(4-aminophenyl) disulfide;bis(3-aminophenyl) disulfide; 2,2′-bis(4-aminonaphthyl) disulfide;2,2′-bis(3-aminonaphthyl) disulfide; 2,2′-bis(4-aminonaphthyl)disulfide; 2,2′-bis(5-aminonaphthyl) disulfide;2,2′-bis(6-aminonaphthyl) disulfide; 2,2′-bis(7-aminonaphthyl)disulfide; 2,2′-bis(8-aminonaphthyl) disulfide;1,1′-bis(2-aminonaphthyl) disulfide; 1,1′-bis(3-aminonaphthyl)disulfide; 1,1′-bis(3-aminonaphthyl) disulfide;1,1′-bis(4-aminonaphthyl) disulfide; 1,1′-bis(5-aminonaphthyl)disulfide; 1,1′-bis(6-aminonaphthyl) disulfide;1,1′-bis(7-aminonaphthyl) disulfide; 1,1′-bis(8-aminonaphthyl)disulfide; 1,2′-diamino-1,2′-dithiodinaphthalene;2,3′-diamino-1,2′-dithiodinaphthalene; bis(4-chlorophenyl) disulfide;bis(2-chlorophenyl) disulfide; bis(3-chlorophenyl) disulfide;bis(4-bromophenyl) disulfide; bis(2-bromophenyl) disulfide;bis(3-bromophenyl) disulfide; bis(4-fluorophenyl) disulfide;bis(4-iodophenyl) disulfide; bis(2,5-dichlorophenyl) disulfide;bis(3,5-dichlorophenyl) disulfide; bis (2,4-dichlorophenyl) disulfide;bis(2,6-dichlorophenyl) disulfide; bis(2,5-dibromophenyl) disulfide;bis(3,5-dibromophenyl) disulfide; bis(2-chloro-5-bromophenyl) disulfide;bis(2,4,6-trichlorophenyl) disulfide; bis(2,3,4,5,6-pentachlorophenyl)disulfide; bis(4-cyanophenyl) disulfide; bis(2-cyanophenyl) disulfide;bis(4-nitrophenyl) disulfide; bis(2-nitrophenyl) disulfide;2,2′-dithiobenzoic acid ethylester; 2,2′-dithiobenzoic acid methylester;2,2′-dithiobenzoic acid; 4,4′-dithiobenzoic acid ethylester;bis(4-acetylphenyl) disulfide; bis(2-acetylphenyl) disulfide;bis(4-formylphenyl) disulfide; bis(4-carbamoylphenyl) disulfide;1,1′-dinaphthyl disulfide; 2,2′-dinaphthyl disulfide; 1,2′-dinaphthyldisulfide; 2,2′-bis(1-chlorodinaphthyl) disulfide;2,2′-bis(1-bromonaphthyl) disulfide; 1,1′-bis(2-chloronaphthyl)disulfide; 2,2′-bis(1-cyanonaphthyl) disulfide;2,2′-bis(1-acetylnaphthyl) disulfide; and the like; or a mixturethereof. Preferred organosulfur components include 4,4′-diphenyldisulfide, 4,4′-ditolyl disulfide, or 2,2′-benzamido diphenyl disulfide,or a mixture thereof. A more preferred organosulfur component includes4,4′-ditolyl disulfide. In another embodiment, metal-containingorganosulfur components can be used according to the invention. Suitablemetal-containing organosulfur components include, but are not limitedto, cadmium, copper, lead, and tellurium analogs ofdiethyldithiocarbamate, diamyldithiocarbamate, anddimethyldithiocarbamate, or mixtures thereof.

Suitable substituted or unsubstituted aromatic organic components thatdo not include sulfur or a metal include, but are not limited to,4,4′-diphenyl acetylene, azobenzene, or a mixture thereof. The aromaticorganic group preferably ranges in size from C₆ to C₂₀, and morepreferably from C₆ to C₁₀. Suitable inorganic sulfide componentsinclude, but are not limited to titanium sulfide, manganese sulfide, andsulfide analogs of iron, calcium, cobalt, molybdenum, tungsten, copper,selenium, yttrium, zinc, tin, and bismuth.

A substituted or unsubstituted aromatic organic compound is alsosuitable as a soft and fast agent. Suitable substituted or unsubstitutedaromatic organic components include, but are not limited to, componentshaving the formula (R₁)_(x)—R₃-M-R₄—(R₂)_(y), wherein R₁ and R₂ are eachhydrogen or a substituted or unsubstituted C₁₋₂₀ linear, branched, orcyclic alkyl, alkoxy, or alkylthio group, or a single, multiple, orfused ring C₆ to C₂₄ aromatic group; x and y are each an integer from 0to 5; R₃ and R₄ are each selected from a single, multiple, or fused ringC₆ to C₂₄ aromatic group; and M includes an azo group or a metalcomponent. R₃ and R₄ are each preferably selected from a C₆ to C₁₀aromatic group, more preferably selected from phenyl, benzyl, naphthyl,benzamido, and benzothiazyl. R₁ and R₂ are each preferably selected froma substituted or unsubstituted C₁₋₁₀ linear, branched, or cyclic alkyl,alkoxy, or alkylthio group or a C₆ to C₁₀ aromatic group. When R₁, R₂,R₃, or R₄, are substituted, the substitution may include one or more ofthe following substituent groups: hydroxy and metal salts thereof;mercapto and metal salts thereof; halogen; amino, nitro, cyano, andamido; carboxyl including esters, acids, and metal salts thereof; silyl;acrylates and metal salts thereof; sulfonyl or sulfonamide; andphosphates and phosphites. When M is a metal component, it may be anysuitable elemental metal available to those of ordinary skill in theart. Typically, the metal will be a transition metal, althoughpreferably it is tellurium or selenium. In one embodiment, the aromaticorganic compound is substantially free of metal, while in anotherembodiment the aromatic organic compound is completely free of metal.

The soft and fast agent can also include a Group VIA component.Elemental sulfur and polymeric sulfur are commercially available fromElastochem, Inc. of Chardon, Ohio Exemplary sulfur catalyst compoundsinclude PB(RM-S)-80 elemental sulfur and PB(CRST)-65 polymeric sulfur,each of which is available from Elastochem, Inc. An exemplary telluriumcatalyst under the tradename TELLOY® and an exemplary selenium catalystunder the tradename VANDEX® are each commercially available from RTVanderbilt.

Other suitable soft and fast agents include, but are not limited to,hydroquinones, benzoquinones, quinhydrones, catechols, and resorcinols.

Suitable hydroquinone compounds include compounds represented by thefollowing formula, and hydrates thereof:

wherein each R₁, R₂, R₃, and R₄ are hydrogen; halogen; alkyl; carboxyl;metal salts thereof, and esters thereof; acetate and esters thereof;formyl; acyl; acetyl; halogenated carbonyl; sulfo and esters thereof;halogenated sulfonyl; sulfino; alkylsulfinyl; carbamoyl; halogenatedalkyl; cyano; alkoxy; hydroxy and metal salts thereof; amino; nitro;aryl; aryloxy; arylalkyl; nitroso; acetamido; or vinyl.

Other suitable hydroquinone compounds include, but are not limited to,hydroquionone; tetrachlorohydroquinone; 2-chlorohydroquionone;2-bromohydroquinone; 2,5-dichlorohydroquinone; 2,5-dibromohydroquinone;tetrabromohydroquinone; 2-methylhydroquinone; 2-t-butylhydroquinone;2,5-di-t-amylhydroquinone; and 2-(2-chlorophenyl) hydroquinone hydrate.

More suitable hydroquinone compounds include compounds represented bythe following formula, and hydrates thereof:

wherein each R₁, R₂, R₃, and R₄ are a metal salt of a carboxyl; acetateand esters thereof; hydroxy; a metal salt of a hydroxy; amino; nitro;aryl; aryloxy; arylalkyl; nitroso; acetamido; or vinyl.

Suitable benzoquinone compounds include compounds represented by thefollowing formula, and hydrates thereof:

wherein each R₁, R₂, R₃, and R₄ are hydrogen; halogen; alkyl; carboxyl;metal salts thereof, and esters thereof; acetate and esters thereof;formyl; acyl; acetyl; halogenated carbonyl; sulfo and esters thereof;halogenated sulfonyl; sulfino; alkylsulfinyl; carbamoyl; halogenatedalkyl; cyano; alkoxy; hydroxy and metal salts thereof; amino; nitro;aryl; aryloxy; arylalkyl; nitroso; acetamido; or vinyl.

Other suitable benzoquinone compounds include one or more compoundsrepresented by the following formula, and hydrates thereof:

wherein each R₁, R₂, R₃, and R₄ are a metal salt of a carboxyl; acetateand esters thereof; hydroxy; a metal salt of a hydroxy; amino; nitro;aryl; aryloxy; arylalkyl; nitroso; acetamido; or vinyl.

Suitable quinhydrones include one or more compounds represented by thefollowing formula, and hydrates thereof:

wherein each R₁, R₂, R₃, R₄, R₅, R₆, R₇, and R₈ are hydrogen; halogen;alkyl; carboxyl; metal salts thereof, and esters thereof; acetate andesters thereof; formyl; acyl; acetyl; halogenated carbonyl; sulfo andesters thereof; halogenated sulfonyl; sulfino; alkylsulfinyl; carbamoyl;halogenated alkyl; cyano; alkoxy; hydroxy and metal salts thereof;amino; nitro; aryl; aryloxy; arylalkyl; nitroso; acetamido; or vinyl.

Other suitable quinhydrones include those having the above formula,wherein each R₁, R₂, R₃, R₄, R₅, R₆, R₇, and R₈ are a metal salt of acarboxyl; acetate and esters thereof; hydroxy; a metal salt of ahydroxy; amino; nitro; aryl; aryloxy; arylalkyl; nitroso; acetamido; orvinyl. Suitable catechols include one or more compounds represented bythe following formula, and hydrates thereof:

wherein each R₁, R₂, R₃, and R₄ are hydrogen; halogen; alkyl; carboxyl;metal salts thereof, and esters thereof; acetate and esters thereof;formyl; acyl; acetyl; halogenated carbonyl; sulfo and esters thereof;halogenated sulfonyl; sulfino; alkylsulfinyl; carbamoyl; halogenatedalkyl; cyano; alkoxy; hydroxy and metal salts thereof; amino; nitro;aryl; aryloxy; arylalkyl; nitroso; acetamido; or vinyl.

Suitable resorcinols include one or more compounds represented by thefollowing formula, and hydrates thereof:

wherein each R₁, R₂, R₃, and R₄ are hydrogen; halogen; alkyl; carboxyl;metal salts thereof, and esters thereof; acetate and esters thereof;formyl; acyl; acetyl; halogenated carbonyl; sulfo and esters thereof;halogenated sulfonyl; sulfino; alkylsulfinyl; carbamoyl; halogenatedalkyl; cyano; alkoxy; hydroxy and metal salts thereof; amino; nitro;aryl; aryloxy; arylalkyl; nitroso; acetamido; or vinyl.

Fillers may also be added to the thermoset rubber composition of thecore to adjust the density of the composition, up or down. Typically,fillers include materials such as tungsten, zinc oxide, barium sulfate,silica, calcium carbonate, zinc carbonate, metals, metal oxides andsalts, regrind (recycled core material typically ground to about 30 meshparticle), high-Mooney-viscosity rubber regrind, trans-regrind corematerial (recycled core material containing high trans-isomer ofpolybutadiene), and the like. When trans-regrind is present, the amountof trans-isomer is preferably between about 10% and about 60%. In apreferred embodiment of the invention, the core comprises polybutadienehaving a cis-isomer content of greater than about 95% and trans-regrindcore material (already vulcanized) as a filler. Any particle sizetrans-regrind core material is sufficient, but is preferably less thanabout 125 μm.

Fillers added to one or more portions of the golf ball typically includeprocessing aids or compounds to affect rheological and mixingproperties, density-modifying fillers, tear strength, or reinforcementfillers, and the like. The fillers are generally inorganic, and suitablefillers include numerous metals or metal oxides, such as zinc oxide andtin oxide, as well as barium sulfate, zinc sulfate, calcium carbonate,barium carbonate, clay, tungsten, tungsten carbide, an array of silicas,and mixtures thereof. Fillers may also include various foaming agents orblowing agents which may be readily selected by one of ordinary skill inthe art. Fillers may include polymeric, ceramic, metal, and glassmicrospheres may be solid or hollow, and filled or unfilled. Fillers aretypically also added to one or more portions of the golf ball to modifythe density thereof to conform to uniform golf ball standards. Fillersmay also be used to modify the weight of the center or at least oneadditional layer for specialty balls, e.g., a lower weight ball ispreferred for a player having a low swing speed.

Materials such as tungsten, zinc oxide, barium sulfate, silica, calciumcarbonate, zinc carbonate, metals, metal oxides and salts, and regrind(recycled core material typically ground to about 30 mesh particle) arealso suitable fillers.

The polybutadiene and/or any other base rubber or elastomer system mayalso be foamed, or filled with hollow microspheres or with expandablemicrospheres which expand at a set temperature during the curing processto any low specific gravity level. Other ingredients such as sulfuraccelerators, e.g., tetramethylthiuram di, tri, or tetrasulfide, and/ormetal-containing organosulfur components may also be used according tothe invention. Suitable metal-containing organosulfur acceleratorsinclude, but are not limited to, cadmium, copper, lead, and telluriumanalogs of diethyldithiocarbamate, diamyldithiocarbamate, anddimethyldithiocarbamate, or mixtures thereof. Other ingredients such asprocessing aids e.g., fatty acids and/or their metal salts, processingoils, dyes and pigments, as well as other additives known to one skilledin the art may also be used in the present invention in amountssufficient to achieve the purpose for which they are typically used.

Without being bound by theory, it is believed that the percentage ofdouble bonds in the trans configuration may be manipulated throughout acore containing at least one main-chain unsaturated rubber (i.e.,polybutadiene), plastic, or elastomer resulting in a trans gradient. Thetrans gradient may be influenced (up or down) by changing the type andamount of cis-to-trans catalyst (or soft-and-fast agent), the type andamount of peroxide, and the type and amount of coagent in theformulation. For example, a formulation containing about 0.25 phr ZnPCTPmay have a trans gradient of about 5% across the core whereas aformulation containing about 2 phr ZnPCTP may have a trans gradient ofabout 10%, or higher. The trans gradient may also be manipulated throughthe cure times and temperatures. It is believed that lower temperaturesand shorter cure times yield lower trans gradients, although acombination of many of these factors may yield gradients of differingand/or opposite directions from that resulting from use of a singlefactor.

The % trans isomer in a core can also be manipulated by addingorganosulfur compounds, such as those listed above, to the coreformulation including but not limited to pentachlorothiophenol, zincpentachlorothiophenol, ditolyl disulfide, and diphenyl disulfide. Theamount of the organosulfur compound and the overall state of cure affectthe amount of the trans isomer that is produced during the curereaction. Another method of increasing the trans content in a core is tointroduce an unsaturated rubber that contains a high level of transisomer, such as high trans containing polybutadiene or high transcontaining polyoctenamer into the core formulation. High trans rubbercan be used with or without the organosulfur compounds.

In general, higher and/or faster cure rates tend to yield higher levelsof trans content, as do higher concentrations of peroxides,soft-and-fast agents, and, to some extent, ZDA concentration. Even thetype of rubber may have an effect on trans levels, with those catalyzedby rare-earth metals, such as Nd, being able to form higher levels oftrans polybutadiene compared to those rubbers formed from Group VIIImetals, such as Co, Ni, and Li.

The measurement of trans-isomer content of polybutadiene referred toherein was and can be accomplished as follows. Calibration standards areprepared using at least two polybutadiene rubber samples of known transcontent, e.g., high and low percent trans-polybutadiene). These samplesare used alone and blended together in such a way as to create a ladderof trans-polybutadiene content of at least about 1.5% to 50% or tobracket the unknown amount, such that the resulting calibration curvecontains at least about 13 equally-spaced points.

Using a commercially-available FTIR spectrometer equipped with aPhotoacoustic (“PAS”) cell, a PAS spectrum of each standard was obtainedusing the following instrument parameters: scan at speed of 2.5 KHz(0.16 cm/s optical velocity), use a 1.2 KHz electronic filter, set anundersampling ratio of 2 (number of laser signal zero crossings beforecollecting a sample), co-add a minimum of 128 scans at a resolution of 4cm⁻¹ over a range of 375 to 4000 cm⁻¹ with a sensitivity setting of 1.

The cis-, trans-, and vinyl-polybutadiene peaks are found between600-1100 cm⁻¹ in the PAS spectrum. The area under each of thetrans-polybutadiene peaks can be integrated. Determining the fraction ofeach peak area relative to the total area of the three isomer peaksallow construction of a calibration curve of the trans-polybutadienearea fraction versus the actual trans-polybutadiene content. Thecorrelation coefficient (R²) of the resulting calibration curve must bea minimum of 0.95.

A PAS spectrum is obtained, using the parameters described above, forthe unknown core material at the point of interest (e.g., the surface orcenter of the core) by filling the PAS cell with a sample containing afreshly cut, uncontaminated surface free of foreign matters, such asmold release and the like. The trans-polybutadiene area fraction of theunknown is analyzed to determine the actual trans-isomer content fromthe calibration curve.

In one known circumstance when barium sulfate is included, the abovemethod for testing trans-content may be less accurate. Thus, anadditional or alternative test of the trans-content of polybutadiene isas follows. Calibration standards are prepared using at least twopolybutadienes of known trans-content (e.g., high and low percenttrans-polybutadiene). These samples are used alone and blended togetherin such a way as to create a ladder of trans-polybutadiene content of atleast about 1.5% to 50% or to bracket the unknown amount, such that theresulting calibration curve contains at least about 13 equally-spacedpoints.

Using an FT-Raman spectrometer equipped with a near-infrared laser, aStokes Raman spectrum should be obtained from each standard using thefollowing instrument parameters: sufficient laser power (typically400-800 mW) to obtain good signal-to-noise ratio without causingexcessive heating or fluorescence; a resolution of 2 cm⁻¹; over a Ramanshift spectral range of 400-4000 cm⁻¹; and co-adding at least 300 scans.

A calibration curve may be constructed from the data generated above,using a chemometrics approach and software, such as PLSplus/IQ fromGalactic Industries Corp. An acceptable calibration was obtained withthis software using a PLS-1 curve generated using an SNV (detrend)pathlength correction, a mean center data preparation, and a 5-point SGsecond derivative over the spectral range of 1600-1700 cm⁻¹. Thecorrelation coefficient (R²) of the resulting calibration curve must beat least 0.95.

Preferably, the geometric center of the core has a trans content ofabout 1% to 10%, more preferably 2% to 9%, and most preferably 4% to 8%.The outer surface of the core preferably has a trans content of about 5%to 15%, more preferably about 7% to 12%, and most preferably about 8% to10%. A half-radius point in the core preferably has a trans content ofabout 5% to 16%, more preferably about 6% to 14%, and most preferablyabout 7% to 13%.

Cores most suitable for the golf balls of the present invention have anouter surface and a center and are formed from a substantiallyhomogenous rubber composition. An intermediate layer, such as a casinglayer (inner cover), is disposed about the core, and a cover layer isformed around the intermediate layer, the cover typically formed from acastable polyurea or a castable polyurethane (i.e., meaning coverscomprising castable polyurea (100% urea linkages/no urethane linkages);castable polyurethane (100% urethane linkages/no urea linkages);castable hybrid poly(urethane/urea) (the prepolymer is all urethanelinkages and is cured with an amine); and castable hybridpoly(urea/urethane) (the prepolymer is all urea linkages and is curedwith a polyol). In a preferred embodiment, the outer surface of the corehas a trans-polybutadiene content of about 6% to 10%, the center of thecore has a trans-polybutadiene content of about 1% to 3%, and the transcontent of the outer surface of the core is greater than the transcontent of the center by about 6% or greater to define a positive transgradient along the core radius (i.e., the surface trans content ishigher than the center trans content—a core having the oppositedisposition of trans content would be considered to have a negativetrans gradient and is also envisioned herein).

Referring to FIG. 1, in one embodiment of the present invention the golfball 10 includes a core 12, an inner cover layer 14, and an outer coverlayer 16.

The invention preferably includes a core of at least a single layer, thecore having a “low hardness gradient” formulation and cure cycle, hereindefined as a “LOG” core. Such LOG cores may have a “positive” Shore Chardness gradient (defined as the core surface hardness minus thegeometric center hardness) of from about 1 Shore C to about 10 Shore C,more preferably about 2 Shore C to less than 8 Shore C, and mostpreferably about 2.5 Shore C to 7.5 Shore C. Where the LOG core includesmore than one layer, e.g., a center surrounded by an outer core layer,either or both of the layers may have a low (or shallow) “positive”hardness gradient. The balls of the present invention preferably includeat least a LOG core, a cover layer.

Optionally, one or more intermediate layer(s) between the core and coverlayer is present. Most preferably, a single layer LOG core having adiameter of about 1.45 to 1.62 inches, more preferably about 1.50 to1.58 inches, and most preferably about 1.51 to 1.55 inches, is enclosedwith two cover layers. The inner cover layer is preferably formed froman ionomeric material, such as a conventional ionomer or ahighly-/fully-neutralized ionomer, having a hardness of about 60 Shore Dor greater. The outer cover layer is preferably formed frompolyurethane, polyurea, or a blend thereof, or a copolymer or hybridthereof, having a hardness about 60 Shore D or less. In this mostpreferred embodiment, the LOG core has a geometric center hardness ofabout 70 to 80 Shore C, more preferably about 72 to 78 Shore C, and asurface hardness of about 71 to 88 Shore C, more preferably about 73 to80 Shore C, and most preferably, about 74 to 78 Shore C. The LOG corealso preferably has a compression (Atti) of about 50 to 90, morepreferably about 60 to 80, and a surface trans-polybutadiene content ofabout 10% or less and a trans-polybutadiene content at the geometriccenter of about 8% or less.

The optional intermediate layer may be either relatively-hard (Shore Chardness of at least about 85, preferably at least about 90; and a ShoreD hardness of at least about 61, preferably at least about 63) orrelatively-soft (Shore C hardness about 85 or less, preferably about 83or less, and a Shore D hardness of about 61 or less, preferably about 57or less). The intermediate layer may include either a thermoplastic orthermosetting polymeric compositions. Thermosetting compositions includediene rubber based compositions, such as a peroxide or sulfur cured PBR,SBR, EPDM, NR, etc., or a polyurethane, polyurea, epoxy,urethane-acrylate, etc. Thermoplastic materials include ionomers,non-ionomers, polyesters, polyamides, PEBAX or HYTREL TPE's, andstyrene-block copolymers (SBS, SEBS, etc).

Cover materials may be any of the same materials disclosed as suitableintermediate layer materials, and may even include a LOG or zero- orlow-hardness-gradient rubber formulation/cure, but preferably include anionomer, a polyurethane, or a polyurea. The cover layer(s) may be eitherhard or soft, with hardness values as described for the intermediatelayers above.

The single (preferably), multi-layer, or multi-piece core may have adiameter of about 0.80 to 1.62 inches, preferably about 1.1 to 1.62inches, more preferably about 1.2 to 1.60 inches, and most preferablyabout 1.3 to 1.58 inches. The combination of the LOG core and theintermediate layer(s) preferably have an outer diameter of about 1.0 to1.66 inches, more preferably about 1.25 to 1.64 inches, and mostpreferably about 1.40 to 1.62 inches. The cover layer preferably had athickness of about 0.010 to 0.080 inches, more preferably about 0.015 to0.060 inches, and most preferably about 0.020 to 0.040 inches.

Generally when a harder cover is employed, it is typically coupled witha relatively-soft intermediate layer to produce a relativelylow-spinning golf ball. When a softer cover is used, it is typicallycoupled with a relatively hard intermediate layer to produce a golf ballhaving higher spin rates than the same construction using a hard cover.In alternative embodiments, a hard cover can be used with a hardintermediate layer or a soft cover can be used with a soft intermediatelayer. It should be understood that the term ‘hard’ refers to a Shore Dhardness of 55 or greater, the term ‘soft’ refers to a Shore D hardnessof 45 or less; a ‘typical’ hardness would be 45 to 55 Shore D.

In another preferred embodiment the golf ball includes three layers. ALOG core preferably has a shallow “positive” hardness gradient of about1 to 5 Shore C, a diameter of about 1.55 inches, a compression of about50 to 100, preferably 65 to 85, and a COR of about 0.800-0.815,preferably 0.800-0.810. The intermediate layer is formed from athermoplastic ionomer-based composition having a hardness of about 80Shore C and an outer diameter of about 1.60 inches. A single cover layeris formed from an ionomer and has a thickness of about 0.040 inches anda hardness of about 66 Shore D. An alternative low-compression centerembodiment may include a center having a compression of about 1 to 50,more preferably about 10 to 40, most preferably about 15 to 35.

The core of the present invention may also have a Soft Center DeflectionIndex (“SCDI”) compression of less than about 160, more preferably, fromabout 40 and about 160, and most preferably, from about 60 and about120.

The SCDI is a program change for the Dynamic Compression Machine (“DCM”)that allows determination of the pounds required to deflect a core 10%of its diameter. The DCM is an apparatus that applies a load to a coreor ball and measures the number of inches the core or ball is deflectedat measured loads. A crude load/deflection curve is generated that isfit to the Atti compression scale that results in a number beinggenerated that represents an Atti compression. The DCM does this via aload cell attached to the bottom of a hydraulic cylinder that istriggered pneumatically at a fixed rate (typically about 1.0 ft/s)towards a stationary core. Attached to the cylinder is an LVDT thatmeasures the distance the cylinder travels during the testing timeframe.A software-based logarithmic algorithm ensures that measurements are nottaken until at least five successive increases in load are detectedduring the initial phase of the test.

The SCDI is a slight variation of this set up. The hardware is the same,but the software and output has changed. With the SCDI, the interest isin the pounds of force required to deflect a core x amount of inches.That amount of deflection is 10% percent of the core diameter. The DCMis triggered, the cylinder deflects the core by 10% of its diameter, andthe DCM reports back the pounds of force required (as measured from theattached load cell) to deflect the core by that amount. The valuedisplayed is a single number in units of pounds.

Alternatively, the LOG core of the previous embodiment has a diameter ofabout 1.450 inches and is encased with an intermediate layer formed froma non-ionomeric thermoplastic (i.e., a PEBAX-type ester-amide blockcopolymer) having a of about 40 Shore D and an outer diameter of about1.580 inches. A preferred cover layer is an ionomeric cover layer havinga hardness of about 68 Shore D and a thickness of about 0.050 inches.

In still another preferred embodiment, the golf ball includes fourlayers. The core is formed from a LOG formula as described above and hasan outer diameter of about 1.50 inches. The golf ball includes twointermediate layers, each having a thickness of about 0.060 inches. Theinnermost intermediate layer preferably has a hardness of about 45 ShoreD and the outermost intermediate layer has a hardness of about 66 ShoreD. The cover layer has a thickness of about 0.030 inches and is formedfrom a thermosetting, castable polyurethane or polyurea and has ahardness of about 53 Shore D.

In a fourth preferred embodiment, the golf ball includes four layers.The core includes a LOG center having a diameter of about 1.0 inch andan outer core layer. The outer core layer has an outer diameter of about1.540 inches and a surface hardness of about 89 to 92 Shore C. The outercore layer is formed from a “conventional” polybutadiene rubberformulation and not a LOG formula. The intermediate layer is formed froman ionomer-based material and has an outer diameter of about 1.62 inchesand a hardness of about 67 Shore D. The cover is formed from a castpolyurethane or polyurea and has a hardness of about 45 Shore D.

The hardness of the outer surface of the core may be lower than ahardness of the geometric center to define a negative hardness gradientof about −1 to about −15, more preferably about −5 to −10. The outersurface of the core may have a hardness of about 68 to about 80 Shore Cand the geometric center has a hardness of about 68 to 80 Shore C. Ahardness measured at the outer surface of the core may also be greaterthan a hardness of the core center to define a positive hardnessgradient of about 0 to 5, more preferably about 2 to 5. The corepreferably has a diameter of about 1.5 inches to 1.59 inches, morepreferably about 1.51 to 1.57 inches, most preferably about 1.51 inchesto 1.55 inches, including, specifically, 1.51 inches, 1.53 inches, or1.55 inches.

The above embodiments may be tailored to meet predetermined performanceproperties. For example, alternative embodiments include those having aninner core having an outer diameter of about 0.250 inches to about 1.550inches, preferably about 0.500 inches to about 1.500 inches, and morepreferably about 0.750 inches to about 1.400 inches. In preferredembodiments, the inner core has an outer diameter of about 1.000 inch,1.200 inches, or 1.300 inches, with a most preferred outer diameterbeing 1.130 inches. The outer core layer should have an outer diameter(the entire dual core) of about 1.30 inches to about 1.620 inches,preferably 1.400 inches to about 1.600 inches, and more preferably about1.500 inches to about 1.590 inches. In preferred embodiments, the outercore layer has an outer diameter of about 1.510 inches, 1.530 inches, ormost preferably 1.550 inches.

A number of cores were formed based on the formulation and cure cycledescribed in TABLE 1 below and core hardness values are reported inTABLE 2 below.

TABLE 1 Formulation (phr) Ex 1 Ex 2 Ex 3 Comp Ex 1 Comp Ex 2 Comp Ex 3SR-526⁺ 34.0 34.0 31.2 29.0 29.0 29.0 ZnO 5 5 5 5 5 5 BaSO₄ 11.2 11.216.1 13.8 13.8 13.8 VANOX MBPC* 0.40 0.40 0.40 — 0.50 —TRIGONOX-265-50B** 1.4 1.4 1.6 — — 0.8 PERKADOX BC-FF*** — — — 1.0 1.6 —polybutadiene 100 100 100 100 100 100 ZnPCTP 2.35 2.35 2.60 2.35 2.352.35 regrind — — 17 17 — — antioxidant/initiator ratio 0.57 0.57 0.50 —0.31 — Cure Temp. (° F.) 305 315 320 350 335 335 Cure Time (min) 14 1116 11 11 11 Properties diameter (in) 1.530 1.530 1.530 1.530 1.530 1.530Atti compression 69 63 70 69 47 — COR @ 125 ft/s 0.808 0.806 0.804 0.804— — *VANOX ® MBPC: 2,2′-methylene-bis-(4-methyl-6-t-butylphenol)available from R. T. Vanderbilt Company Inc.; **TRIGONOX ® 265-50B: amixture of 1,1-di(t-butylperoxy)-3,3,5-trimethycyclohexane anddi(2-t-butylperoxyisopropyl)benzene 50% active on an inert carrieravailable from Akzo Nobel; ***PERKADOX ® BC-FF: Dicumyl peroxide(99%-100% active) available from Akzo Nobel; and ⁺SR-526: ZDA availablefrom Sartomer

TABLE 2 Shore C Hardness Distance Comp Comp from Center Ex 1 Ex 2 Ex 3Ex 1 Ex 2 Comp Ex 3 Center 73 70 71 61 52 61 2 74 71 72 67 57 62 4 74 7273 70 62 65 6 75 73 73 72 64 67 8 75 73 73 73 64 69 10 75 73 74 73 64 7112 74 74 73 72 66 72 14 74 74 72 73 70 73 16 70 71 70 77 71 73 18 60 6063 80 72 73 Surface 63 70 66 85 73 74 Surface − Center −10 0 −5 24 21 13

A number of cores were prepared and hardness measurements were madeacross the core. The cores, designated as Examples 1-4 in Table 3 below,all had an outer diameter of 1.53 inches. The hardness, in Shore C, wasmeasured according to ASTM D-2240 at various locations across across-section of the core. The hardness results are tabulated below forthe geometric center, outer surface, and at locations 2-mm, 4-mm, 6-mm,8-mm, 10-mm, 12-mm, 14-mm, 16-mm, and 18-mm radially-outward from thegeometric center of the core. The general core formulation included atleast 85 phr CB1221 polybutadiene rubber, at least 15 phr CB23polybutadiene rubber, about 35 phr zinc diacrylate, about 1 phrperoxide, about 0.5 phr VANOX® MBPC, about 5 phr ZnO, about 0.5 phrZnPCTP, about 1 phr AFLUX® 16, about 13 phr POLYWATE® 325, and about 15phr regrind. The cure cycles were adjusted, as necessary, to vary thehardness gradient across the core. Temperature/time criteria variedbetween about 330° F./20 min, 335° F./18 min, 340° F./16 min, and 345°F./14 min.

TABLE 3 Ex A Ex B Ex C Ex D hardness hardness hardness hardness CoreLocation (Shore C) (Shore C) (Shore C) (Shore C) Center 73.1 73.1 72.173.9  2 mm 72.2 72.1 73.2 75.2  4 mm 72.9 73.9 75.3 75.9  6 mm 73.7 74.376.5 77.2  8 mm 74.0 74.3 76.5 77.1 10 mm 74.7 73.8 76.5 76.7 12 mm 73.973.9 77.1 76.8 14 mm 73.9 73.4 76.7 76.7 16 mm 74.3 73.7 76.1 76.5 18 mm74.5 74.3 75.5 76.1 Surface 74.6 75.1 75.7 78.2 Hardness 1.5 2.0 3.6 4.3Gradient

The hardness gradients, as seen in Table 3 above, range from 1.5 to 4.3Shore C. These are shallow positive gradients and are determined bysubtracting the hardness at the geometric center from the hardness atthe surface of the core.

Cores formed from the same general formula as described above are alsotested for the % cis-polybutadiene, % trans-polybutadiene, and %vinyl-polybutadiene. The results are presented below in Table 4.

TABLE 4 Core % trans-PBR % vinyl-PBR % cis-PBR Ex E outer surface 7.62.7 89.6 Ex E half radius 7.8 2.8 89.4 Ex E center 7.4 2.8 89.7 Ex Fouter surface 9.2 2.7 88.4 Ex F half radius 13.2 2.6 85.4 Ex F center8.5 2.9 88.6 Ex G outer surface 8.7 2.8 88.5 Ex G half radius 12.6 2.685.7 Ex G center 7.3 2.6 90.1

Cores E-G, above in TABLE 3, exhibit a shallow trans-polybutadienegradient from core center to the surface.

The surface hardness of a core is obtained from the average of a numberof measurements taken from opposing hemispheres of a core, taking careto avoid making measurements on the parting line of the core or onsurface defects, such as holes or protrusions. Hardness measurements aremade pursuant to ASTM D-2240 “Indentation Hardness of Rubber and Plasticby Means of a Durometer.” Because of the curved surface of a core, caremust be taken to insure that the core is centered under the durometerindentor before a surface hardness reading is obtained. A calibrated,digital durometer, capable of reading to 0.1 hardness units is used forall hardness measurements and is set to take hardness readings at 1second after the maximum reading is obtained. The digital durometer mustbe attached to, and its foot made parallel to, the base of an automaticstand, such that the weight on the durometer and attack rate conform toASTM D-2240.

To prepare a core for hardness gradient measurements, the core is gentlypressed into a hemispherical holder having an internal diameterapproximately slightly smaller than the diameter of the core, such thatthe core is held in place in the hemispherical portion of the holderwhile concurrently leaving the geometric central plane of the coreexposed. The core is secured in the holder by friction, such that itwill not move during the cutting and grinding steps, but the friction isnot so excessive that distortion of the natural shape of the core wouldresult. The core is secured such that the parting line of the core isroughly parallel to the top of the holder. The diameter of the core ismeasured 90 degrees to this orientation prior to securing. A measurementis also made from the bottom of the holder to the top of the core toprovide a reference point for future calculations. A rough cut, madeslightly above the exposed geometric center of the core using a band sawor other appropriate cutting tool, making sure that the core does notmove in the holder during this step. The remainder of the core, still inthe holder, is secured to the base plate of a surface grinding machine.The exposed ‘rough’ core surface is ground to a smooth, flat surface,revealing the geometric center of the core, which can be verified bymeasuring the height of the bottom of the holder to the exposed surfaceof the core, making sure that exactly half of the original height of thecore, as measured above, has been removed to within ±0.004 inches.

Leaving the core in the holder, the center of the core is found with acenter square and carefully marked and the hardness is measured at thecenter mark. Hardness measurements at any distance from the center ofthe core may be measured by drawing a line radially outward from thecenter mark, and measuring and marking the distance from the center,typically in 2-mm increments. All hardness measurements performed on theplane passing through the geometric center are performed while the coreis still in the holder and without having disturbed its orientation,such that the test surface is constantly parallel to the bottom of theholder. The hardness difference from any predetermined location on thecore is calculated as the average surface hardness minus the hardness atthe appropriate reference point, e.g., at the center of the core forsingle, solid core, such that a core surface softer than its center willhave a negative hardness gradient.

Referring to TABLES 1-2, in Example 1, the surface is 10 Shore C pointslower than the center hardness and 12 Shore C points lower than thehardest point in the core. In Example 3, the surface is 5 Shore C pointslower than the center hardness and 8 Shore C points lower than thehardest point in the core. In Example 2, the center and surface hardnessvalues are equal and the softest point in the core is 10 Shore C pointslower than the surface.

In the examples of the invention presented in TABLE 1, the curetemperatures are varied from 305° F. to 320° F. and cure times arevaried from 11 to 16 minutes. The core compositions of examples 1 and 2are identical, and only the cure cycle is changed. In example 3 theamount of antioxidant is identical to examples 1 and 2, but otheringredients are varied as well the cure cycle. Additionally, the ratioof antioxidant to initiator varies from 0.50 to 0.57 from example 1 and2 to example 3.

The ratio of antioxidant to initiator is one factor to control thesurface hardness of the cores. The data shown in TABLE 2 shows thathardness gradient is at least, but not limited to, a function of theamount of antioxidant and peroxide, their ratio, and the cure cycle. Itshould be noted that higher antioxidant also requires higher peroxideinitiator to maintain the desired compression.

The core of Comparative Example 1, whose composition is shown in TABLE 1was cured using a conventional cure cycle, with a cure temperature of350° F. and a cure time of 11 minutes. The inventive cores were producedusing cure cycles of 305° F. for 14 minutes, 315° F. for 11 minutes and320° F. for 16 minutes. The hardness gradients of these cores weremeasured and the following observations can be made. For the cores ofthe Comparative Examples, as expected, a conventional hard surface tosoft center gradient can be clearly seen. The gradients for inventivecores follow substantially the same shape as one another.

In all preferred embodiments of invention, the hardness of the core atthe surface is at most about the same as or substantially less than thehardness of the core at the center. Furthermore, the center hardness ofthe core may not be the hardest point in the core, but in all cases, itis preferred that it is at least equal to or harder than the surface.Additionally, the lowest hardness anywhere in the core does not have tooccur at the surface. In some embodiments, the lowest hardness valueoccurs within about the outer 6 mm of the core surface. However, thelowest hardness value within the core can occur at any point from thesurface, up to, but not including the center, as long as the surfacehardness is still equal to, or less than the hardness of the center. Itshould be noted that in the present invention the formulation is thesame throughout the core, or core layer, and no surface treatment isapplied to the core to obtain the preferred surface hardness.

While the inventive golf ball may be formed from a variety of differingand conventional cover materials (both intermediate layer(s) and outercover layer), preferred cover materials include, but are not limited to:

-   -   (1) Polyurethanes, such as those prepared from polyols or        polyamines and diisocyanates or polyisocyanates and/or their        prepolymers, and those disclosed in U.S. Pat. Nos. 5,334,673 and        6,506,851;    -   (2) Polyureas, such as those disclosed in U.S. Pat. Nos.        5,484,870 and 6,835,794; and    -   (3) Polyurethane-urea hybrids, blends or copolymers comprising        urethane or urea segments.

Suitable polyurethane compositions comprise a reaction product of atleast one polyisocyanate and at least one curing agent. The curing agentcan include, for example, one or more polyamines, one or more polyols,or a combination thereof. The polyisocyanate can be combined with one ormore polyols to form a prepolymer, which is then combined with the atleast one curing agent. Thus, the polyols described herein are suitablefor use in one or both components of the polyurethane material, i.e., aspart of a prepolymer and in the curing agent. Suitable polyurethanes aredescribed in U.S. Patent Application Publication No. 2005/0176523, whichis incorporated by reference in its entirety.

Any polyisocyanate available to one of ordinary skill in the art issuitable for use according to the invention. Exemplary polyisocyanatesinclude, but are not limited to, 4,4′-diphenylmethane diisocyanate(MDI); polymeric MDI; carbodiimide-modified liquid MDI;4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI); p-phenylene diisocyanate(PPDI); m-phenylene diisocyanate (MPDI); toluene diisocyanate (TDI);3,3′-dimethyl-4,4′-biphenylene diisocyanate; isophoronediisocyanate;1,6-hexamethylene diisocyanate (HDI); naphthalene diisocyanate; xylenediisocyanate; p-tetramethylxylene diisocyanate; m-tetramethylxylenediisocyanate; ethylene diisocyanate; propylene-1,2-diisocyanate;tetramethylene-1,4-diisocyanate; cyclohexyl diisocyanate;dodecane-1,12-diisocyanate; cyclobutane-1,3-diisocyanate;cyclohexane-1,3-diisocyanate; cyclohexane-1,4-diisocyanate;1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane; methylcyclohexylene diisocyanate; triisocyanate of HDI; triisocyanate of2,4,4-trimethyl-1,6-hexane diisocyanate; tetracene diisocyanate;napthalene diisocyanate; anthracene diisocyanate; isocyanurate oftoluene diisocyanate; uretdione of hexamethylene diisocyanate; andmixtures thereof. Polyisocyanates are known to those of ordinary skillin the art as having more than one isocyanate group, e.g.,di-isocyanate, tri-isocyanate, and tetra-isocyanate. Preferably, thepolyisocyanate includes MDI, PPDI, TDI, or a mixture thereof, and morepreferably, the polyisocyanate includes MDI. It should be understoodthat, as used herein, the term MDI includes 4,4′-diphenylmethanediisocyanate, polymeric MDI, carbodiimide-modified liquid MDI, andmixtures thereof and, additionally, that the diisocyanate employed maybe “low free monomer,” understood by one of ordinary skill in the art tohave lower levels of “free” monomer isocyanate groups, typically lessthan about 0.1% free monomer isocyanate groups. Examples of “low freemonomer” diisocyanates include, but are not limited to Low Free MonomerMDI, Low Free Monomer TDI, and Low Free Monomer PPDI.

The at least one polyisocyanate should have less than about 14%unreacted NCO groups. Preferably, the at least one polyisocyanate has nogreater than about 8.0% NCO, more preferably no greater than about 7.8%,and most preferably no greater than about 7.5% NCO with a level of NCOof about 7.2 or 7.0, or 6.5% NCO commonly used.

Any polyol available to one of ordinary skill in the art is suitable foruse according to the invention. Exemplary polyols include, but are notlimited to, polyether polyols, hydroxy-terminated polybutadiene(including partially/fully hydrogenated derivatives), polyester polyols,polycaprolactone polyols, and polycarbonate polyols. In one preferredembodiment, the polyol includes polyether polyol. Examples include, butare not limited to, polytetramethylene ether glycol (PTMEG),polyethylene propylene glycol, polyoxypropylene glycol, and mixturesthereof. The hydrocarbon chain can have saturated or unsaturated bondsand substituted or unsubstituted aromatic and cyclic groups. Preferably,the polyol of the present invention includes PTMEG.

In another embodiment, polyester polyols are included in thepolyurethane material. Suitable polyester polyols include, but are notlimited to, polyethylene adipate glycol; polybutylene adipate glycol;polyethylene propylene adipate glycol; o-phthalate-1,6-hexanediol;poly(hexamethylene adipate) glycol; and mixtures thereof. Thehydrocarbon chain can have saturated or unsaturated bonds, orsubstituted or unsubstituted aromatic and cyclic groups.

In another embodiment, polycaprolactone polyols are included in thematerials of the invention. Suitable polycaprolactone polyols include,but are not limited to, 1,6-hexanediol-initiated polycaprolactone,diethylene glycol initiated polycaprolactone, trimethylol propaneinitiated polycaprolactone, neopentyl glycol initiated polycaprolactone,1,4-butanediol-initiated polycaprolactone, and mixtures thereof. Thehydrocarbon chain can have saturated or unsaturated bonds, orsubstituted or unsubstituted aromatic and cyclic groups.

In yet another embodiment, polycarbonate polyols are included in thepolyurethane material of the invention. Suitable polycarbonates include,but are not limited to, polyphthalate carbonate and poly(hexamethylenecarbonate) glycol. The hydrocarbon chain can have saturated orunsaturated bonds, or substituted or unsubstituted aromatic and cyclicgroups. In one embodiment, the molecular weight of the polyol is fromabout 200 to about 4000.

Polyamine curatives are also suitable for use in the polyurethanecomposition of the invention and have been found to improve cut, shear,and impact resistance of the resultant balls. Preferred polyaminecuratives include, but are not limited to,3,5-dimethylthio-2,4-toluenediamine and isomers thereof;3,5-diethyltoluene-2,4-diamine and isomers thereof, such as3,5-diethyltoluene-2,6-diamine;4,4′-bis-(sec-butylamino)-diphenylmethane;1,4-bis-(sec-butylamino)-benzene, 4,4′-methylene-bis-(2-chloroaniline);4,4′-methylene-bis-(3-chloro-2,6-diethylaniline);polytetramethyleneoxide-di-p-aminobenzoate; N,N′-dialkyldiamino diphenylmethane; p,p′-methylene dianiline; m-phenylenediamine;4,4′-methylene-bis-(2-chloroaniline);4,4′-methylene-bis-(2,6-diethylaniline);4,4′-methylene-bis-(2,3-dichloroaniline);4,4′-diamino-3,3′-diethyl-5,5′-dimethyl diphenylmethane; 2,2′,3,3′-tetrachloro diamino diphenylmethane; trimethylene glycoldi-p-aminobenzoate; and mixtures thereof. Preferably, the curing agentof the present invention includes 3,5-dimethylthio-2,4-toluenediamineand isomers thereof, such as ETHACURE® 300, commercially available fromAlbermarle Corporation of Baton Rouge, La. Suitable polyamine curatives,which include both primary and secondary amines, preferably havemolecular weights ranging from about 64 to about 2000.

At least one of a diol, triol, tetraol, or hydroxy-terminated curativesmay be added to the aforementioned polyurethane composition. Suitablediol, triol, and tetraol groups include ethylene glycol; diethyleneglycol; polyethylene glycol; propylene glycol; polypropylene glycol;lower molecular weight polytetramethylene ether glycol;1,3-bis(2-hydroxyethoxy) benzene; 1,3-bis-[2-(2-hydroxyethoxy) ethoxy]benzene; 1,3-bis-{2-[2-(2-hydroxyethoxy) ethoxy] ethoxy} benzene;1,4-butanediol; 1,5-pentanediol; 1,6-hexanediol;resorcinol-di-(β-hydroxyethyl) ether; hydroquinone-di-(β-hydroxyethyl)ether; and mixtures thereof. Preferred hydroxy-terminated curativesinclude 1,3-bis(2-hydroxyethoxy) benzene; 1,3-bis-[2-(2-hydroxyethoxy)ethoxy] benzene; 1,3-bis-{2-[2-(2-hydroxyethoxy) ethoxy] ethoxy}benzene; 1,4-butanediol, and mixtures thereof. Preferably, thehydroxy-terminated curatives have molecular weights ranging from about48 to 2000. It should be understood that molecular weight, as usedherein, is the absolute weight average molecular weight and would beunderstood as such by one of ordinary skill in the art.

Both the hydroxy-terminated and amine curatives can include one or moresaturated, unsaturated, aromatic, and cyclic groups. Additionally, thehydroxy-terminated and amine curatives can include one or more halogengroups. The polyurethane composition can be formed with a blend ormixture of curing agents. If desired, however, the polyurethanecomposition may be formed with a single curing agent.

In a preferred embodiment of the present invention, saturatedpolyurethanes are used to form one or more of the cover layers,preferably the outer cover layer, and may be selected from among bothcastable thermoset and thermoplastic polyurethanes.

In this embodiment, the saturated polyurethanes of the present inventionare substantially free of aromatic groups or moieties. Saturatedpolyurethanes suitable for use in the invention are a product of areaction between at least one polyurethane prepolymer and at least onesaturated curing agent. The polyurethane prepolymer is a product formedby a reaction between at least one saturated polyol and at least onesaturated diisocyanate. As is well known in the art, that a catalyst maybe employed to promote the reaction between the curing agent and theisocyanate and polyol, or the curing agent and the prepolymer.

Saturated diisocyanates which can be used include, without limitation,ethylene diisocyanate; propylene-1,2-diisocyanate;tetramethylene-1,4-diisocyanate; 1,6-hexamethylene-diisocyanate (HDI);2,2,4-trimethylhexamethylene diisocyanate; 2,4,4-trimethylhexamethylenediisocyanate; dodecane-1,12-diisocyanate; dicyclohexylmethanediisocyanate; cyclobutane-1,3-diisocyanate;cyclohexane-1,3-diisocyanate; cyclohexane-1,4-diisocyanate;1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane; isophoronediisocyanate; methyl cyclohexylene diisocyanate; triisocyanate of HDI;triisocyanate of 2,2,4-trimethyl-1,6-hexane diisocyanate. The mostpreferred saturated diisocyanates are 4,4′-dicyclohexylmethanediisocyanate and isophorone diisocyanate.

Saturated polyols which are appropriate for use in this inventioninclude without limitation polyether polyols such as polytetramethyleneether glycol and poly(oxypropylene) glycol. Suitable saturated polyesterpolyols include polyethylene adipate glycol, polyethylene propyleneadipate glycol, polybutylene adipate glycol, polycarbonate polyol andethylene oxide-capped polyoxypropylene diols. Saturated polycaprolactonepolyols which are useful in the invention include diethyleneglycol-initiated polycaprolactone, 1,4-butanediol-initiatedpolycaprolactone, 1,6-hexanediol-initiated polycaprolactone; trimethylolpropane-initiated polycaprolactone, neopentyl glycol initiatedpolycaprolactone, and polytetramethylene ether glycol-initiatedpolycaprolactone. The most preferred saturated polyols arepolytetramethylene ether glycol and PTMEG-initiated polycaprolactone.

Suitable saturated curatives include 1,4-butanediol, ethylene glycol,diethylene glycol, polytetramethylene ether glycol, propylene glycol;trimethanolpropane; tetra-(2-hydroxypropyl)-ethylenediamine; isomers andmixtures of isomers of cyclohexyldimethylol, isomers and mixtures ofisomers of cyclohexane bis(methylamine); triisopropanolamine; ethylenediamine; diethylene triamine; triethylene tetramine; tetraethylenepentamine; 4,4′-dicyclohexylmethane diamine;2,2,4-trimethyl-1,6-hexanediamine; 2,4,4-trimethyl-1,6-hexanediamine;diethyleneglycol di-(aminopropyl)ether;4,4′-bis-(sec-butylamino)-dicyclohexylmethane;1,2-bis-(sec-butylamino)cyclohexane; 1,4-bis-(sec-butylamino)cyclohexane; isophorone diamine; hexamethylene diamine; propylenediamine; 1-methyl-2,4-cyclohexyl diamine; 1-methyl-2,6-cyclohexyldiamine; 1,3-diaminopropane; dimethylamino propylamine; diethylaminopropylamine; imido-bis-propylamine; isomers and mixtures of isomers ofdiaminocyclohexane; monoethanolamine; diethanolamine; triethanolamine;monoisopropanolamine; and diisopropanolamine. The most preferredsaturated curatives are 1,4-butanediol, 1,4-cyclohexyldimethylol and4,4′-bis-(sec-butylamino)-dicyclohexylmethane.

Alternatively, other suitable polymers include partially or fullyneutralized ionomer, metallocene, or other single-site catalyzedpolymer, polyester, polyamide, non-ionomeric thermoplastic elastomer,copolyether-esters, copolyether-amides, polycarbonate, polybutadiene,polyisoprene, polystryrene block copolymers (such asstyrene-butadiene-styrene), styrene-ethylene-propylene-styrene,styrene-ethylene-butylene-styrene, and the like, and blends thereof.Thermosetting polyurethanes or polyureas are suitable for the outercover layers of the golf balls of the present invention.

Additionally, polyurethane can be replaced with or blended with apolyurea material. Polyureas are distinctly different from polyurethanecompositions, but also result in desirable aerodynamic and aestheticcharacteristics when used in golf ball components. The polyurea-basedcompositions are preferably saturated in nature.

Without being bound to any particular theory, it is now believed thatsubstitution of the long chain polyol segment in the polyurethaneprepolymer with a long chain polyamine oligomer soft segment to form apolyurea prepolymer, improves shear, cut, and resiliency, as well asadhesion to other components. Thus, the polyurea compositions of thisinvention may be formed from the reaction product of an isocyanate andpolyamine prepolymer crosslinked with a curing agent. For example,polyurea-based compositions of the invention may be prepared from atleast one isocyanate, at least one polyether amine, and at least onediol curing agent or at least one diamine curing agent.

Any polyamine available to one of ordinary skill in the art is suitablefor use in the polyurea prepolymer. Polyether amines are particularlysuitable for use in the prepolymer. As used herein, “polyether amines”refer to at least polyoxyalkyleneamines containing primary amino groupsattached to the terminus of a polyether backbone. Due to the rapidreaction of isocyanate and amine, and the insolubility of many ureaproducts, however, the selection of diamines and polyether amines islimited to those allowing the successful formation of the polyureaprepolymers. In one embodiment, the polyether backbone is based ontetramethylene, propylene, ethylene, trimethylolpropane, glycerin, andmixtures thereof.

Suitable polyether amines include, but are not limited to,methyldiethanolamine; polyoxyalkylenediamines such as,polytetramethylene ether diamines, polyoxypropylenetriamine, andpolyoxypropylene diamines; poly(ethylene oxide capped oxypropylene)ether diamines; propylene oxide-based triamines;triethyleneglycoldiamines; trimethylolpropane-based triamines;glycerin-based triamines; and mixtures thereof. In one embodiment, thepolyether amine used to form the prepolymer is JEFFAMINE® D2000(manufactured by Huntsman Chemical Co. of Austin, Tex.).

The molecular weight of the polyether amine for use in the polyureaprepolymer may range from about 100 to about 5000. In one embodiment,the polyether amine molecular weight is about 200 or greater, preferablyabout 230 or greater. In another embodiment, the molecular weight of thepolyether amine is about 4000 or less. In yet another embodiment, themolecular weight of the polyether amine is about 600 or greater. Instill another embodiment, the molecular weight of the polyether amine isabout 3000 or less. In yet another embodiment, the molecular weight ofthe polyether amine is between about 1000 and about 3000, and morepreferably is between about 1500 to about 2500. Because lower molecularweight polyether amines may be prone to forming solid polyureas, ahigher molecular weight oligomer, such as JEFFAMINE® D2000, ispreferred.

As briefly discussed above, some amines may be unsuitable for reactionwith the isocyanate because of the rapid reaction between the twocomponents. In particular, shorter chain amines are fast reacting. Inone embodiment, however, a hindered secondary diamine may be suitablefor use in the prepolymer. Without being bound to any particular theory,it is believed that an amine with a high level of stearic hindrance,e.g., a tertiary butyl group on the nitrogen atom, has a slower reactionrate than an amine with no hindrance or a low level of hindrance. Forexample, 4,4′-bis-(sec-butylamino)-dicyclohexylmethane (CLEARLINK® 1000)may be suitable for use in combination with an isocyanate to form thepolyurea prepolymer.

Any isocyanate available to one of ordinary skill in the art is suitablefor use in the polyurea prepolymer. Isocyanates for use with the presentinvention include aliphatic, cycloaliphatic, araliphatic, aromatic, anyderivatives thereof, and combinations of these compounds having two ormore isocyanate (NCO) groups per molecule. The isocyanates may beorganic polyisocyanate-terminated prepolymers. The isocyanate-containingreactable component may also include any isocyanate-functional monomer,dimer, trimer, or multimeric adduct thereof, prepolymer,quasi-prepolymer, or mixtures thereof. Isocyanate-functional compoundsmay include monoisocyanates or polyisocyanates that include anyisocyanate functionality of two or more.

Suitable isocyanate-containing components include diisocyanates havingthe generic structure: O═C═N—R—N═C═O, where R is preferably a cyclic,aromatic, or linear or branched hydrocarbon moiety containing from about1 to about 20 carbon atoms. The diisocyanate may also contain one ormore cyclic groups or one or more phenyl groups. When multiple cyclic oraromatic groups are present, linear and/or branched hydrocarbonscontaining from about 1 to about 10 carbon atoms can be present asspacers between the cyclic or aromatic groups. In some cases, the cyclicor aromatic group(s) may be substituted at the 2-, 3-, and/or4-positions, or at the ortho-, meta-, and/or para-positions,respectively. Substituted groups may include, but are not limited to,halogens, primary, secondary, or tertiary hydrocarbon groups, or amixture thereof.

Examples of diisocyanates that can be used with the present inventioninclude, but are not limited to, substituted and isomeric mixturesincluding 2,2′-, 2,4′-, and 4,4′-diphenylmethane diisocyanate;3,3′-dimethyl-4,4′-biphenylene diisocyanate; toluene diisocyanate;polymeric MDI; carbodiimide-modified liquid 4,4′-diphenylmethanediisocyanate; para-phenylene diisocyanate; meta-phenylene diisocyanate;triphenyl methane-4,4′- and triphenyl methane-4,4′-triisocyanate;naphthylene-1,5-diisocyanate; 2,4′-, 4,4′-, and 2,2-biphenyldiisocyanate; polyphenyl polymethylene polyisocyanate; mixtures of MDIand PMDI; mixtures of PMDI and TDI; ethylene diisocyanate;propylene-1,2-diisocyanate; tetramethylene-1,2-diisocyanate;tetramethylene-1,3-diisocyanate; tetramethylene-1,4-diisocyanate;1,6-hexamethylene-diisocyanate; octamethylene diisocyanate;decamethylene diisocyanate; 2,2,4-trimethylhexamethylene diisocyanate;2,4,4-trimethylhexamethylene diisocyanate; dodecane-1,12-diisocyanate;cyclobutane-1,3-diisocyanate; cyclohexane-1,2-diisocyanate;cyclohexane-1,3-diisocyanate; cyclohexane-1,4-diisocyanate;methyl-cyclohexylene diisocyanate; 2,4-methylcyclohexane diisocyanate;2,6-methylcyclohexane diisocyanate; 4,4′-dicyclohexyl diisocyanate;2,4′-dicyclohexyl diisocyanate; 1,3,5-cyclohexane triisocyanate;isocyanatomethylcyclohexane isocyanate;1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane;isocyanatoethylcyclohexane isocyanate; bis(isocyanatomethyl)-cyclohexanediisocyanate; 4,4′-bis(isocyanatomethyl) dicyclohexane;2,4′-bis(isocyanatomethyl) dicyclohexane; isophorone diisocyanate;triisocyanate of HDI; triisocyanate of 2,2,4-trimethyl-1,6-hexanediisocyanate; 4,4′-dicyclohexylmethane diisocyanate;2,4-hexahydrotoluene diisocyanate; 2,6-hexahydrotoluene diisocyanate;1,2-, 1,3-, and 1,4-phenylene diisocyanate; aromatic aliphaticisocyanate, such as 1,2-, 1,3-, and 1,4-xylene diisocyanate;meta-tetramethylxylene diisocyanate; para-tetramethylxylenediisocyanate; trimerized isocyanurate of any polyisocyanate, such asisocyanurate of toluene diisocyanate, trimer of diphenylmethanediisocyanate, trimer of tetramethylxylene diisocyanate, isocyanurate ofhexamethylene diisocyanate, isocyanurate of isophorone diisocyanate, andmixtures thereof; dimerized uredione of any polyisocyanate, such asuretdione of toluene diisocyanate, uretdione of hexamethylenediisocyanate, and mixtures thereof; modified polyisocyanate derived fromthe above isocyanates and polyisocyanates; and mixtures thereof.

Examples of saturated diisocyanates that can be used with the presentinvention include, but are not limited to, ethylene diisocyanate;propylene-1,2-diisocyanate; tetramethylene diisocyanate;tetramethylene-1,4-diisocyanate; 1,6-hexamethylene-diisocyanate;octamethylene diisocyanate; decamethylene diisocyanate;2,2,4-trimethylhexamethylene diisocyanate; 2,4,4-trimethylhexamethylenediisocyanate; dodecane-1,12-diisocyanate; cyclobutane-1,3-diisocyanate;cyclohexane-1,2-diisocyanate; cyclohexane-1,3-diisocyanate;cyclohexane-1,4-diisocyanate; methyl-cyclohexylene diisocyanate;2,4-methylcyclohexane diisocyanate; 2,6-methylcyclohexane diisocyanate;4,4′-dicyclohexyl diisocyanate; 2,4′-dicyclohexyl diisocyanate;1,3,5-cyclohexane triisocyanate; isocyanatomethylcyclohexane isocyanate;1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane;isocyanatoethylcyclohexane isocyanate; bis(isocyanatomethyl)-cyclohexanediisocyanate; 4,4′-bis(isocyanatomethyl) dicyclohexane;2,4′-bis(isocyanatomethyl) dicyclohexane; isophorone diisocyanate;triisocyanate of HDI; triisocyanate of 2,2,4-trimethyl-1,6-hexanediisocyanate; 4,4′-dicyclohexylmethane diisocyanate;2,4-hexahydrotoluene diisocyanate; 2,6-hexahydrotoluene diisocyanate;and mixtures thereof. Aromatic aliphatic isocyanates may also be used toform light stable materials. Examples of such isocyanates include 1,2-,1,3-, and 1,4-xylene diisocyanate; meta-tetramethylxylene diisocyanate;para-tetramethylxylene diisocyanate; trimerized isocyanurate of anypolyisocyanate, such as isocyanurate of toluene diisocyanate, trimer ofdiphenylmethane diisocyanate, trimer of tetramethylxylene diisocyanate,isocyanurate of hexamethylene diisocyanate, isocyanurate of isophoronediisocyanate, and mixtures thereof; dimerized uredione of anypolyisocyanate, such as uretdione of toluene diisocyanate, uretdione ofhexamethylene diisocyanate, and mixtures thereof; modifiedpolyisocyanate derived from the above isocyanates and polyisocyanates;and mixtures thereof. In addition, the aromatic aliphatic isocyanatesmay be mixed with any of the saturated isocyanates listed above for thepurposes of this invention.

The number of unreacted NCO groups in the polyurea prepolymer ofisocyanate and polyether amine may be varied to control such factors asthe speed of the reaction, the resultant hardness of the composition,and the like. For instance, the number of unreacted NCO groups in thepolyurea prepolymer of isocyanate and polyether amine may be less thanabout 14 percent. In one embodiment, the polyurea prepolymer has fromabout 5 percent to about 11 percent unreacted NCO groups, and even morepreferably has from about 6 to about 9.5 percent unreacted NCO groups.In one embodiment, the percentage of unreacted NCO groups is about 3percent to about 9 percent. Alternatively, the percentage of unreactedNCO groups in the polyurea prepolymer may be about 7.5 percent or less,and more preferably, about 7 percent or less. In another embodiment, theunreacted NCO content is from about 2.5 percent to about 7.5 percent,and more preferably from about 4 percent to about 6.5 percent.

When formed, polyurea prepolymers may contain about 10 percent to about20 percent by weight of the prepolymer of free isocyanate monomer. Thus,in one embodiment, the polyurea prepolymer may be stripped of the freeisocyanate monomer. For example, after stripping, the prepolymer maycontain about 1 percent or less free isocyanate monomer. In anotherembodiment, the prepolymer contains about 0.5 percent by weight or lessof free isocyanate monomer.

The polyether amine may be blended with additional polyols to formulatecopolymers that are reacted with excess isocyanate to form the polyureaprepolymer. In one embodiment, less than about 30 percent polyol byweight of the copolymer is blended with the saturated polyether amine.In another embodiment, less than about 20 percent polyol by weight ofthe copolymer, preferably less than about 15 percent by weight of thecopolymer, is blended with the polyether amine. The polyols listed abovewith respect to the polyurethane prepolymer, e.g., polyether polyols,polycaprolactone polyols, polyester polyols, polycarbonate polyols,hydrocarbon polyols, other polyols, and mixtures thereof, are alsosuitable for blending with the polyether amine. The molecular weight ofthese polymers may be from about 200 to about 4000, but also may be fromabout 1000 to about 3000, and more preferably are from about 1500 toabout 2500.

The polyurea composition can be formed by crosslinking the polyureaprepolymer with a single curing agent or a blend of curing agents. Thecuring agent of the invention is preferably an amine-terminated curingagent, more preferably a secondary diamine curing agent so that thecomposition contains only urea linkages. In one embodiment, theamine-terminated curing agent may have a molecular weight of about 64 orgreater. In another embodiment, the molecular weight of the amine-curingagent is about 2000 or less. As discussed above, certainamine-terminated curing agents may be modified with a compatibleamine-terminated freezing point depressing agent or mixture ofcompatible freezing point depressing agents.

Suitable amine-terminated curing agents include, but are not limited to,ethylene diamine; hexamethylene diamine; 1-methyl-2,6-cyclohexyldiamine; tetrahydroxypropylene ethylene diamine; 2,2,4- and2,4,4-trimethyl-1,6-hexanediamine;4,4′-bis-(sec-butylamino)-dicyclohexylmethane;1,4-bis-(sec-butylamino)-cyclohexane;1,2-bis-(sec-butylamino)-cyclohexane; derivatives of4,4′-bis-(sec-butylamino)-dicyclohexylmethane; 4,4′-dicyclohexylmethanediamine; 1,4-cyclohexane-bis-(methylamine);1,3-cyclohexane-bis-(methylamine); diethylene glycol di-(aminopropyl)ether; 2-methylpentamethylene-diamine; diaminocyclohexane; diethylenetriamine; triethylene tetramine; tetraethylene pentamine; propylenediamine; 1,3-diaminopropane; dimethylamino propylamine; diethylaminopropylamine; dipropylene triamine; imido-bis-propylamine;monoethanolamine, diethanolamine; triethanolamine; monoisopropanolamine,diisopropanolamine; isophoronediamine;4,4′-methylenebis-(2-chloroaniline); 3,5;dimethylthio-2,4-toluenediamine; 3,5-dimethylthio-2,6-toluenediamine;3,5-diethylthio-2,4-toluenediamine; 3,5; diethylthio-2,6-toluenediamine;4,4′-bis-(sec-butylamino)-diphenylmethane and derivatives thereof;1,4-bis-(sec-butylamino)-benzene; 1,2-bis-(sec-butylamino)-benzene;N,N′-dialkylamino-diphenylmethane; N,N,N′,N′-tetrakis (2-hydroxypropyl)ethylene diamine; trimethyleneglycol-di-p-aminobenzoate;polytetramethyleneoxide-di-p-aminobenzoate;4,4′-methylenebis-(3-chloro-2,6-diethyleneaniline);4,4′-methylenebis-(2,6-diethylaniline); meta-phenylenediamine;paraphenylenediamine; and mixtures thereof. In one embodiment, theamine-terminated curing agent is4,4′-bis-(sec-butylamino)-dicyclohexylmethane.

Suitable saturated amine-terminated curing agents include, but are notlimited to, ethylene diamine; hexamethylene diamine;1-methyl-2,6-cyclohexyl diamine; tetrahydroxypropylene ethylene diamine;2,2,4- and 2,4,4-trimethyl-1,6-hexanediamine;4,4′-bis-(sec-butylamino)-dicyclohexylmethane;1,4-bis-(sec-butylamino)-cyclohexane;1,2-bis-(sec-butylamino)-cyclohexane; derivatives of4,4′-bis-(sec-butylamino)-dicyclohexylmethane; 4,4′-dicyclohexylmethanediamine; 4,4′-methylenebis-(2,6-diethylaminocyclohexane;1,4-cyclohexane-bis-(methylamine); 1,3-cyclohexane-bis-(methylamine);diethylene glycol di-(aminopropyl) ether;2-methylpentamethylene-diamine; diaminocyclohexane; diethylene triamine;triethylene tetramine; tetraethylene pentamine; propylene diamine;1,3-diaminopropane; dimethylamino propylamine; diethylamino propylamine;imido-bis-propylamine; monoethanolamine, diethanolamine;triethanolamine; monoisopropanolamine, diisopropanolamine;isophoronediamine; triisopropanolamine; and mixtures thereof. Inaddition, any of the polyether amines listed above may be used as curingagents to react with the polyurea prepolymers.

Cover layers of the inventive golf ball may also be formed fromionomeric polymers, preferably highly-neutralized ionomers (HNP). In apreferred embodiment, at least one intermediate layer of the golf ballis formed from an HNP material or a blend of HNP materials. The acidmoieties of the HNP's, typically ethylene-based ionomers, are preferablyneutralized greater than about 70%, more preferably greater than about90%, and most preferably at least about 100%. The HNP's can be also beblended with a second polymer component, which, if containing an acidgroup, may be neutralized in a conventional manner, by the organic fattyacids of the present invention, or both. The second polymer component,which may be partially or fully neutralized, preferably comprisesionomeric copolymers and terpolymers, ionomer precursors,thermoplastics, polyamides, polycarbonates, polyesters, polyurethanes,polyureas, thermoplastic elastomers, polybutadiene rubber, balata,metallocene-catalyzed polymers (grafted and non-grafted), single-sitepolymers, high-crystalline acid polymers, cationic ionomers, and thelike. HNP polymers typically have a material hardness of between about20 and about 80 Shore D, and a flexural modulus of between about 3,000psi and about 200,000 psi.

In one embodiment of the present invention the HNP's are ionomers and/ortheir acid precursors that are preferably neutralized, either fully orpartially, with organic acid copolymers or the salts thereof. The acidcopolymers are preferably α-olefin, such as ethylene, C₃₋₈α,β-ethylenically unsaturated carboxylic acid, such as acrylic andmethacrylic acid, copolymers. They may optionally contain a softeningmonomer, such as alkyl acrylate and alkyl methacrylate, wherein thealkyl groups have from 1 to 8 carbon atoms.

The acid copolymers can be described as E/X/Y copolymers where E isethylene, X is an α,β-ethylenically unsaturated carboxylic acid, and Yis a softening comonomer. In a preferred embodiment, X is acrylic ormethacrylic acid and Y is a C₁₋₈ alkyl acrylate or methacrylate ester. Xis preferably present in an amount from about 1 to about 35 weightpercent of the polymer, more preferably from about 5 to about 30 weightpercent of the polymer, and most preferably from about 10 to about 20weight percent of the polymer. Y is preferably present in an amount fromabout 0 to about 50 weight percent of the polymer, more preferably fromabout 5 to about 25 weight percent of the polymer, and most preferablyfrom about 10 to about 20 weight percent of the polymer.

Specific acid-containing ethylene copolymers include, but are notlimited to, ethylene/acrylic acid/n-butyl acrylate, ethylene/methacrylicacid/n-butyl acrylate, ethylene/methacrylic acid/iso-butyl acrylate,ethylene/acrylic acid/iso-butyl acrylate, ethylene/methacrylicacid/n-butyl methacrylate, ethylene/acrylic acid/methyl methacrylate,ethylene/acrylic acid/methyl acrylate, ethylene/methacrylic acid/methylacrylate, ethylene/methacrylic acid/methyl methacrylate, andethylene/acrylic acid/n-butyl methacrylate. Preferred acid-containingethylene copolymers include, ethylene/methacrylic acid/n-butyl acrylate,ethylene/acrylic acid/n-butyl acrylate, ethylene/methacrylic acid/methylacrylate, ethylene/acrylic acid/ethyl acrylate, ethylene/methacrylicacid/ethyl acrylate, and ethylene/acrylic acid/methyl acrylatecopolymers. The most preferred acid-containing ethylene copolymers are,ethylene/(meth) acrylic acid/n-butyl, acrylate, ethylene/(meth)acrylicacid/ethyl acrylate, and ethylene/(meth) acrylic acid/methyl acrylatecopolymers.

Ionomers are typically neutralized with a metal cation, such as Li, Na,Mg, K, Ca, or Zn. It has been found that by adding sufficient organicacid or salt of organic acid, along with a suitable base, to the acidcopolymer or ionomer, however, the ionomer can be neutralized, withoutlosing processability, to a level much greater than for a metal cation.Preferably, the acid moieties are neutralized greater than about 80%,preferably from 90-100%, most preferably 100% without losingprocessability. This accomplished by melt-blending an ethyleneα,β-ethylenically unsaturated carboxylic acid copolymer, for example,with an organic acid or a salt of organic acid, and adding a sufficientamount of a cation source to increase the level of neutralization of allthe acid moieties (including those in the acid copolymer and in theorganic acid) to greater than 90%, (preferably greater than 100%).

The organic acids of the present invention are aliphatic, mono- ormulti-functional (saturated, unsaturated, or multi-unsaturated) organicacids. Salts of these organic acids may also be employed. The salts oforganic acids of the present invention include the salts of barium,lithium, sodium, zinc, bismuth, chromium, cobalt, copper, potassium,strontium, titanium, tungsten, magnesium, cesium, iron, nickel, silver,aluminum, tin, or calcium, salts of fatty acids, particularly stearic,behenic, erucic, oleic, linoelic or dimerized derivatives thereof. It ispreferred that the organic acids and salts of the present invention berelatively non-migratory (they do not bloom to the surface of thepolymer under ambient temperatures) and non-volatile (they do notvolatilize at temperatures required for melt-blending).

The ionomers of the invention may also be more conventional ionomers,i.e., partially-neutralized with metal cations. The acid moiety in theacid copolymer is neutralized about 1 to about 90%, preferably at leastabout 20 to about 75%, and more preferably at least about 40 to about70%, to form an ionomer, by a cation such as lithium, sodium, potassium,magnesium, calcium, barium, lead, tin, zinc, aluminum, or a mixturethereof.

In a preferred embodiment, the inventive single-layer core is enclosedwith two cover layers, where the inner cover layer has a thickness ofabout 0.01 inches to about 0.06 inches, more preferably about 0.015inches to about 0.040 inches, and most preferably about 0.02 inches toabout 0.035 inches, and the inner cover layer is formed from apartially- or fully-neutralized ionomer having a Shore D hardness ofgreater than about 55, more preferably greater than about 60, and mostpreferably greater than about 65. In this embodiment, the outer coverlayer should have a thickness of about 0.015 inches to about 0.055inches, more preferably about 0.02 inches to about 0.04 inches, and mostpreferably about 0.025 inches to about 0.035 inches, and has a hardnessof about Shore D 60 or less, more preferably 55 or less, and mostpreferably about 52 or less. The inner cover layer should be harder thanthe outer cover layer. In this embodiment the outer cover layercomprises a partially- or fully-neutralized iononomer, a polyurethane,polyurea, or blend thereof. A most preferred outer cover layer is acastable or reaction injection molded polyurethane, polyurea orcopolymer or hybrid thereof having a Shore D hardness of about 40 toabout 50. A most preferred inner cover layer material is apartially-neutralized ionomer comprising a zinc, sodium or lithiumneutralized ionomer such as SURLYN® 8940, 8945, 9910, 7930, 7940, orblend thereof having a Shore D hardness of about 63 to about 68.

In another multi-layer cover, single core embodiment, the outer coverand inner cover layer materials and thickness are the same but, thehardness range is reversed, that is, the outer cover layer is harderthan the inner cover layer.

In an alternative preferred embodiment, the golf ball is a one-piecegolf ball having a dimpled surface and having a surface hardness equalto or less than the center hardness (i.e., a negative hardnessgradient). The one-piece ball preferably has a diameter of about 1.680inches to about 1.690 inches, a weight of about 1.620 oz, an Atticompression of from about 40 to 120, and a COR of about 0.750 to 0.825.

In a preferred two-piece ball embodiment, the single-layer core having anegative hardness gradient is enclosed with a single layer of covermaterial having a Shore D hardness of from about 20 to about 80, morepreferably about 40 to about 75 and most preferably about 45 to about70, and comprises a thermoplastic or thermosetting polyurethane,polyurea, polyamide, polyester, polyester elastomer, polyether-amide orpolyester-amide, partially or fully neutralized ionomer, polyolefin suchas polyethylene, polypropylene, polyethylene copolymers such asethylene-butyl acrylate or ethylene-methyl acrylate, poly(ethylenemethacrylic acid) co- and terpolymers, metallocene-catalyzed polyolefinsand polar-group functionalized polyolefins and blends thereof. Apreferred cover material in the two-piece embodiment is an ionomer(either conventional or HNP) having a hardness of about 50 to about 70Shore D. Another preferred cover material in the two-piece embodiment isa thermoplastic or thermosetting polyurethane or polyurea. A preferredionomer is a high acid ionomer comprising a copolymer of ethylene andmethacrylic or acrylic acid and having an acid content of at least 16 toabout 25 weight percent. In this case the reduced spin contributed bythe relatively rigid high acid ionomer may be offset to some extent bythe spin-increasing negative gradient core. The core may have a diameterof about 1.0 inch to about 1.64 inches, preferably about 1.30 inches toabout 1.620, and more preferably about 1.40 inches to about 1.60 inches.

A preferred ionomer for the cover layers of the invention is a high acidionomer comprising a copolymer of ethylene and methacrylic or acrylicacid and having an acid content of at least 16 to about 25 weightpercent (wt. %). As used herein, the terms “high-acid ionomer” and“high-acid copolymer” are defined as an acid copolymer or its ionomerderivatives which contains at least about 16 wt. % acid, preferably fromabout 16 wt. % to about 25 wt. % acid, more preferably from about 18 wt.% to about 22 wt. % acid and most preferably about 18.5 wt. % acid toabout 21 wt. % acid, where about 10 to 90 percent of the acid ispartially-neutralized with sodium, manganese, lithium, potassium, zinc,magnesium, calcium or nickel ions. High-acid ionomers arecommercially-available from DuPont under the tradename SURLYN®. Examplesof suitable high acid ionomers include SURLYN® 8546 (neutralized with Lications), SURLYN® 8150 (neutralized with Na cations), SURLYN® 9120(neutralized with Zn cations), and SURLYN® 9150 (neutralized with Zncations)—each of these SURLYN® materials comprises about 19 wt. %methacrylic acid. Other suitable high-acid copolymers include thosecommercially-available from A. Schulman under the tradename CLARIX®, andhigh-acid ionomers neutralized using Dow 5800 Series ethylene-acrylicacid.

In a preferred embodiment, the cover is a two-layer structure includingan inner cover layer and an outer cover layer. The outer cover comprisesa polyurethane (or polyurea) material as described above. The innercover layer, or casing layer, comprises a high-acid ionomer materialdescribed above. The high-acid ionomers of the invention preferablycomprise greater than about 16 wt. % acid, more preferably greater thanabout 18 wt. % acid, and most preferably about 18.5 wt. % acid to about21 wt. % acid. The most preferred blends include blends of Li/Na/Znhigh-acid ionomers and blends of Na/Zn high-acid ionomers. In a fewpreferred embodiments, the casing layer can comprise a 50/50 blend ofSURLYN® 8546 and SURLYN® 9150, a 50/50 blend of SURLYN® 8150 and SURLYN®9120, or a 75/25 blend of AD 8546 and AD 9945 (a low-acid ionomer).Other blends of high-acid ionomers and low-acid ionomers are envisioned.

In these embodiments, the outer cover layer preferably comprises apolyurethane material that has an isocyanate (NCO) content of about 5percent to about 7 percent NCO, more preferably about 5.5 percent toabout 6.6 percent NCO, and most preferably about 5.9 percent to about6.2 percent NCO. The outer cover has a material (button) hardness ofabout 35 Shore D to about 55 Shore D, more preferably about 38 Shore Dto about 52 Shore D, and most preferably about 40 Shore D or 42 Shore Dto about 46 Shore D to 48 Shore D. In one embodiment, the covercomprises about 3 wt. % to about 5 wt. % of a UV stabilizer, preferablya TINUVIN® UV stabilizer, such as TINUVIN® 328. More preferably the UVstabilizer is present at about 3.8 wt. % to about 4.2 wt. %.

Other suitable ionomers include those listed in TABLE 5 below.

TABLE 5 Mole % Melt Flow Shore D Cation wt % acid Neutralization IndexHardness Ionomers SURLYN ® 7940 Lithium 15.0 MAA 42 2.6 68 SURLYN ® 8940Sodium 15.0 MAA 30 2.8 65 SURLYN ® 8945 Sodium 15.0 MAA 51 4.5 65SURLYN ® 9910 Zinc 15.0 MAA 61 0.7 65 SURLYN ® 9945 Zinc 15.0 MAA 54 4.255 SURLYN ® 6120 Magnesium 19.0 MAA 39 1.1 67 SURLYN ® 8140 Sodium 19.0MAA 37 2.6 68 SURLYN ® 8150 Sodium 19.0 MAA 45 4.5 68 SURLYN ® 9120 Zinc19.0 MAA 36 1.3 66 SURLYN ® 9150 Zinc 19.0 MAA 38 4.5 65 SURLYN ® AD8546 Lithium 19.0 MAA 44 1.1 70 SURLYN ® AD 1092 Magnesium 19.0 MAA —2.8 67 CLARIX ® 211702-01 Zinc 17.0 AA — 2.0 66 CLARIX ® 411701-01Magnesium 17.0 AA — 2.0 66 CLARIX ® 11710-01 Sodium 17.0 AA — 10.0 67CLARIX ® 511704-01 Lithium 17.0 AA — 3.5 67 CLARIX ® 611701-01 Potassium17.0 AA — 1.0 67 Acid Copolymers PRIMACOR ® 5985 NA 20.5 AA 0 240 —PRIMACOR ® 5986 NA 20.5 AA 0 300 — PRIMACOR ® 5980i NA 20.5 AA 0 300 —PRIMACOR ® 5990i NA 20.0 AA 0 1,300 — Dow XUS ® 60751.17 NA 19.8 AA 0600 — Dow XUS ® 60753.02L NA 17.0 AA 0 60 — Melt Flow Index is measuredat 190° C. and the units are g/10 min Shore D hardness is materialhardness measured on a 10-day aged button SURLYN ® materials arecommercially available from DuPont, CLARIX ® resins are commerciallyavailable from A. Schulman, and PRIMACOR ® acid copolymers and XUS ®copolymers are sold by Dow Chemical Co.

The core may have any diameter, but is about 1.0 inch to about 1.64inches, preferably about 1.30 inches to about 1.620, more preferablyabout 1.40 inches to about 1.60 inches, and most preferably about 1.50inches to about 1.56 inches, such as 1.51 inches, 1.52 inches, 1.53inches, 1.54 inches, or 1.5 inches. The core may have a positivehardness gradient but most preferably has a zero or negative hardnessgradient.

Another preferred cover material comprises a castable or reactioninjection moldable polyurethane, polyurea, or copolymer or hybrid ofpolyurethane/polyurea. Preferably, this cover is thermosetting but maybe a thermoplastic, having a Shore D hardness of about 20 to about 70,more preferably about 30 to about 65 and most preferably about 35 toabout 60. A moisture vapor barrier layer, such as disclosed in U.S. Pat.Nos. 6,632,147; 6,932,720; 7,004,854; and 7,182,702, all of which areincorporated by reference herein in their entirety, are optionallyemployed between the cover layer and the core.

While any of the embodiments herein may have any known dimple number andpattern, a preferred number of dimples is 252 to 456, and morepreferably is 330 to 392. The dimples may comprise any width, depth, andedge angle disclosed in the prior art and the patterns may comprisesmultitudes of dimples having different widths, depths and edge angles.The parting line configuration of said pattern may be either a straightline or a staggered wave parting line (SWPL). More preferably the dimplenumber is 330, 332, or 392 and comprises 5 to 7 dimples sizes and theparting line is a SWPL. In a most preferred embodiment, the dimplepattern has 352 dimples arranged in a tetrahedron pattern and theparting line is a SWPL.

In any of these embodiments the single-layer core may be replaced with a2 or more layer core wherein at least one core layer has a negativehardness gradient.

Other than in the operating examples, or unless otherwise expresslyspecified, all of the numerical ranges, amounts, values and percentagessuch as those for amounts of materials and others in the specificationmay be read as if prefaced by the word “about” even though the term“about” may not expressly appear with the value, amount or range.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and attached claims are approximationsthat may vary depending upon the desired properties sought to beobtained by the present invention. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of the claims, each numerical parameter should at least beconstrued in light of the number of reported significant digits and byapplying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Furthermore, when numerical ranges ofvarying scope are set forth herein, it is contemplated that anycombination of these values inclusive of the recited values may be used.

While it is apparent that the illustrative embodiments of the inventiondisclosed herein fulfill the objective stated above, it is appreciatedthat numerous modifications and other embodiments may be devised bythose skilled in the art. Therefore, it will be understood that theappended claims are intended to cover all such modifications andembodiments, which would come within the spirit and scope of the presentinvention.

What is claimed is:
 1. A golf ball comprising: a single core having anouter surface and a geometric center and being formed from asubstantially homogenous rubber composition; an inner cover layerdisposed about the core, the inner cover comprising a high-acid ionomerand having a material hardness of about 66 to 75 Shore D, the high-acidionomer having an acid content of about 16% or greater; and an outercover layer disposed about the inner cover layer, the outer covercomprising a polyurethane and having a material hardness of about 38Shore D to about 56 Shore D; wherein the core surface hardness is from 0Shore C to 15 Shore C lower than the geometric center hardness to definea hardness gradient.
 2. The golf ball of claim 1, wherein the ionomerhas an acid content of about 18.5% to about 21.5%.
 3. The golf ball ofclaim 1, wherein the core has an outer diameter of about 1.45 to about1.62 inches.
 4. The golf ball of claim 3, wherein the core has an outerdiameter of about 1.50 to about 1.58 inches.
 5. The golf ball of claim1, wherein the hardness of the core geometric center is about 72 to 78Shore C.
 6. The golf ball of claim 1, wherein the hardness of the coresurface is about 73 to 80 Shore C.
 7. The golf ball of claim 6, whereinthe core surface hardness is about 74 to 78 Shore C.
 8. The golf ball ofclaim 1, wherein the hardness of the outer surface of the core is 1Shore C to 10 Shore C lower than the hardness at the geometric center.9. The golf ball of claim 8, wherein the hardness of the outer surfaceof the core is 1 Shore C to 5 Shore C lower than the hardness at thegeometric center.
 10. The golf ball of claim 1, wherein the geometriccenter of the core has a hardness of about 60 to 80 Shore C.
 11. Thegolf ball of claim 1, wherein the geometric center of the core has atrans content of about 1% to 10% and the outer surface of the core has atrans content of about 5% to 15%.
 12. The golf ball of claim 11, whereinthe geometric center of the core has a trans content of about 2% to 9%.13. The golf ball of claim 12, wherein the geometric center of the corehas a trans content of about 4% to 8%.
 14. The golf ball of claim 13,wherein the outer surface of the core has a trans content of about 7% to12%.
 15. The golf ball of claim 14, wherein the outer surface of thecore has a trans content of about 8% to 10%.
 16. The golf ball of claim1, wherein a half-radius point located between the geometric center andthe outer surface of the core has a trans content of about 5% to 16%.17. The golf ball of claim 16, wherein the half-radius point has a transcontent of about 7% to 13%.